(c)2000 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)1 Sponsoring Organization.

AGO-37912

AIAA 2000-3057

Prospects of MHD Flow Control For Hypersonlcs

J. T. Llneberry and R. J. RosaLyTec LLCTullahoma, TN

V. A. Bityurin, A. N. Botcharov, and V. G. PotebnyaIVTANMoscow, Russia

35th Intersoclety Energy Conversion EngineeringConference

24-28 July 2000Las Vegas, Nevada

For permission to copy or to republish, contact the American Institute of Aeronautics and Astronautics,1801 Alexander Bell Drive, Suite 500, Reston, VA, 20191-4344.

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PROSPECTS OF MHD FLOW CONTROL FOR HYPERSONICS/. T. Lineberry, R. J. RosaLyTecLLC, Tullahoma,TN

V. A. Bityurin, A. N. Botcharov, V. G. PotebnyaIVTAN, Moscow, Russia

Abstract

The concept of MHD flow control and other MHDapplications for hypersonics vehicles are explored ina generic sense. Plausible MHD interactions with anincoming ionized hypersonic free stream flow arediscussed in relation to their providing means forcontrolling vehicle aerodynamics and propulsion. AnMHD flow control inlet configuration concept and itsoperation are qualitatively described. The prospectsof active MHD boundary layer control and leadingedge heat transfer management with MHD are alsodemonstrated.

Introduction

Recent national initiatives in hypersonics extendfrom a first generation hypersonic cruiser with visiontoward hypersonic accelerator type vehicles, such asthe trans-atmospheric vehicle (aero-space plane) [1].NASA's hypersonic programs are centered aroundtheir need for a single-stage-to-orbit (SSTO) vehicleto realize economical access to space. Similarmission statements can be expressed for commercialenterprises and foreign aeronautic programs, such asthe Japanese HOPE and European FESTIP [1].

Regardless of the mission, all hypersonic initiativesshare the requirement to operate through the air-breathing corridor and a common need for air-breathing engines. An overview of the combined airbreathing propulsion requirements as given in Figure1 identifies the turbojet for low speed acceleration(take-off and landing), transition to a ramjet to Mach4 to 7, transition to a scramjet for acceleration tohypervelocity.

The hypersonic cruiser mission ascends to twentymiles or more to loiter at Mach 6 to 12 range. Theaero-space plane mission extends beyond, to push theenvelope of the scramjet to Mach 20 plus, and withminimal rocket assist, enter low earth orbit aroundone hundreds miles up.

The opportunity that for magnetohydrodynamic (MHD)or magnetogasdynamic (MOD) flow control and otherMHD processes offer hypersonics can be viewed within

Altitude(1,000 ft)r

300

200

10 15 20Velocity (1,000 Ws)

25 30

the context of the missions. The fundamental premisefor utilization of plasma assist (PA), MHD or MOD isthe requirement for an air plasma. Figure 2 showsdefinition of different regimes across the flightenvelope wherein PA/MHD/MGD interaction can betaken advantage of. It also identifies what is required tomake the air plasma available and what type enablingtechnology can be realized.

Pre-lonizationPlasma

FreestreamPlasma

(VSL Operation)

Flow ControlPower Generation

Scramjet OptimizationAerodynamic ControlThermal Management

Flow Control ——Power Generation _

Aerodynamic Control ~

10 15 20Velocity (1,000 ft/sec)

25 30

Figure 2. MHD Potential in Different Flight Regimes

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An assessment can be made that in the lowerhypervelocity regime (M<7), MHD flow control canbe realized to a moderate degree by seeding the flowaround the aircraft with an easily ionizable element(e.g., K, Cs) [2,3]. In the moderate hypervelocityregime (7 < M < 12) use of pre-ionization (such asposed using RF, microwaves or e-beam excitation[4,5] is a possibility for enabling strong MHD/MGDinteractions. This regime is consistent with thehypersonic cruiser's mission, hi the far hypervelocityregion (M> 12 to 15) the incoming flow willnaturally be highly ionized due solely to thedynamics of the flight condition. In this regime, thewhole gambit of strong MHD interactions is plausibleto enable the whole array of advanced flightconcepts. This region is consistent with the extendedportion of aero-space plane's mission.

The opportunity for MHD flow and related MHDideas as an enabling technology to enhanceperformance is applicable to both the hypersoniccruiser and the aero-space plane missions. Theopportunity for the latter is most obvious fromFigures 1 and Figure 2 when applied to control offlow capture and/or conditioning. MHD inlet airinduction augmentation and design point control canprovide a non-intrusive means to push the scramjetenvelope well into the reach of boost-to-orbit,minimizing the rocket requirement. Furthermore, theaero-space plane (as currently envisioned) must havea large inlet area relative to the body cross-section inorder to facilitate the thrust margin needed forsufficient acceleration to reach orbital speed. Theincorporation of an MHD controlled inlet (as a virtualinlet) can decrease this size requirement enablingdrag loss in the air-breathing corridor to be signi-ficantly reduced.

The hypersonic cruiser requires no thrust margin atcruise speeds. Its optimum design criteria ismaximum lift-to-drag at design point subject tospecific volume to planform area constraints. Theregime of operation for the cruiser is on the "edge" ofMHD assist viability. Contemporary designs, such asBoeing's Mach 10 cruiser", [6] have scoped a vehiclecapable of achieving the cruiser mission. Imple-mentation of an MHD inlet for flow control andaugmentation into the cruiser is plausible. Drawingfrom the results of Figure 2 we recognize that thisimplementation would burden the cruiser design withthe additional requirement of a pre-ionization systemin this moderate hypersonic flight regime. However,with this implemented, the cruiser mission can beexpanded to higher Mach number operationenhancing its performance and range.

The opportunity that the MHD inlet and its relatedelements offer to the cruiser is that of extending itscapability to higher velocities on demand. HigherMach number operation implies increased range andexpansion of the cruiser's "global reach" capability.

Other aspects of MHD control, such as, MHD/MGDboundary layer control and thermal management, areadditional opportunities for an improved vehicledesign that are applicable to both the cruiser and theaero-space plane.

MHD Applications and Flow Control Concepts

In the following subsections we provide a review ofseveral contemporary ideas for use of MGD/MHD toimprove hypersonic vehicles performance and range.These ideas are presented in general terms with somebackground analysis provided for judgment.

MHD Power GenerationUtilization of MHD for production of auxiliary poweron-board a hypersonic craft has been proposed. Thepotential for MHD power in hypersonic flightregimes exists by utilization of the plasma producedaround the aircraft as a result of the hypervelocityflight dynamics or by use of an external ionizationsystem.

It is recognized that there is tremendous potential forMHD power generation in hypervelocity flight. Theideal power density parameter is defined as:

where a is the plasma electrical conductivity, u is theflight velocity, and B is the magnetic field intensity.This parameter is a measure of the power per unitvolume possible with MHD.

Power density can be extremely high in hypersonicflight by virtue of the hypervelocity condition. Figure3 graphs calculations of this parameter and showsthat inlet MHD power densities from hundreds tothousands of megawatts per cubic meter of volumeare plausible.

Ideas for means MHD power production over bothexternal and internal surfaces have been tabled.External systems utilize the flow over the aircraftsbody bounded by the shock systems as the generatorvolume. Power is extracted with surface electrodes.

Internal concepts draft the free stream plasma flowinto an MHD generator channel. The most prominent

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MHD Power Density Parameter (MW/m 3)3,000

2,000

1,000

0 { mho/raj;'* fjf

''•^•'KfiW;. 100

10 15Flight Velocity (ft/s)

20 25

Figure 3. Ideal Hypersonic Inlet Power DensityParameter Variation

of the internal concepts is that which utilizes thepropulsion system inlet as the generator channel.This idea is part of the scramjet energy by-passscheme under evaluation in the Russian AJAXconcept.

It can be concluded that configuring the inlet as adiagonal MHD generator is optimum and necessarybecause of the high Hall parameter condition ( 8 < COT< 100 depending on flight regime). The diagonalgenerator configuration provides optimum electricalperformance and also operational flexibility.

Figure 4 shows a plausible inlet MHD generatorconcept. The sketch shows envisioned features of thediagonally configured inlet. The vector diagram inthe inlet plane exemplifies the MHD induced EMFwhich is always directed upward for the B-Fieldorientation chosen. Typical diagonal inter-electrodeconnection is noted and the power consolidationcircuit is depicted (end-to-end connection across thelength of the generator).

General electrical operation of the diagonal generatoris shown in the power diagram of Figure 5. Detailsof the power diagram can be found in references [11].The diagram shows a diagonal MHD generatorleadline that extends across concentric circles thatrepresent discrete power levels. The magnitude ofcurrent densities at different points along the loadlineis graphed as the abscissa and ordinates, i.e., Hallcurrent, Jx, and Faraday current, Jy.

The inset in Figure 4 provides definition of theelectrical vectors at the different operating points

JxEV HMD Row Expansion * Jx x B

B Jy4 _ .JxxB

JxMHD Row Compression

B

JxJy Neutralized

Figure 4. Illustration of Inlet MHD Generator

MHD Generator Power Diagram

*^<r

Flow Compression

Ex ~ Hall Electric FieldEy ~ Faraday Electric Fieldjx - Hall Current Densityjy ~ Faraday Current Densityd> ~ Tan Diag Angle (Ey/Ex)P - Hall Parameter

• Load Line• Open Circuit© Max Power° Short Circuit• 3 jy Neutralized

Figure 5. Typical MHD Power Diagram

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cited in Figure 5. The Lorentz force (J x B) producedby the generator electrodynamics acts directly on the

, flow to redistribute the flow and produce pressuregradients across the flowfield. The Lorentz bodyforce action due to Jy manifests itself to the aircraftas a drag force. That force due to Jx provides alateral displacement of the flow toward the channelwalls whose direction is dependent on the directionof the Jx vector (which depends on the loadingcondition as can be seen on the loadline of Figure 5)

Utilization of the MUD generator as a means tocontrol flow through the inlet and propulsion systemis inferred from the discussion above. Control of theJx vector magnitude and direction can provide ameans to either compress or expand of the flow awayfrom the wall. If the wall is the vehicle forebody, thisaction can control the draft into the inlet. Work on theMHD inlet utilizing this type of approach is currentlyunder research both in the US and abroad.

The MHD Flow Control

The concept of MHD flow control arose from studiesundertook to evaluate the benefits of using plasmaeffects and MHD interactions to positively modifyshock structure and flowfield around various surfacesof the hypersonic vehicle [2-10]. It was recognizedthat improvements in these areas could significantlyenhance vehicle performance (aerodynamics andpropulsion) and simplify the mechanical design of thevehicle subsystems.

hi Figure 6 are sketches of a generic hypersonicvehicle in different views. On these sketches arehighlighted areas on the vehicle wherein MGD/MHDmight be employed.

Area 1 denotes leading edges (i.e., cowl lip, wings,nose and tail leading edge). In these areas, MHD orMOD can be designed to modify/control the shockstandoff and reduce thermal load.

Area 2 is the entrance region of the inlet. In thisregion MHD can be used, through active control overLorentz body forces, to either reduce or augment airinduction and flowfield structure; thereby, enhancinginlet capture performance, operational flexibility, andproviding control of shock on lip design point.

Area 3 encompasses the forebody of the aircraft butis also representative of internal surfaces such aswithin the inlet. In general, positive boundary layercontrol using MHD interaction is very practical and itcan be used to control separation, reduce thermalconvective load, and reduce skin friction drag.

As a first look at MHD flow control, we extend thediscussion of the previous section on the MHD inlet.Figure 7 is an artist's sketch of two scenarios for off-design operation. These illustrations show the fore-body/inlet with bow shock position and flow stream-lines.

GENERAL HYPERSONIC VEHICLE CONCEPT

1 ~ Leading Edge Control Areas2 ~ Influenced Area for MHD Inlet Flow Contro3 ~ Forebody MHD Boundary Layer Control

Figure 6. Areas on the Hypersonic Vehicle Where MHD Flow Control Concepts Can Enhance Performance

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w/o MHDBow Shock

Spillage

MHD AssistShock-on-Lip

w/o MHDBow Shock

Ingested

MHD AssistShock-On-Lip

Scenario 1 Scenario 2Figure 7. Illustration of MHD Inlet Control Over Design Point

Scenario 1 is flight at lower than design Machnumber the shock stands off the lip and spillage dragexists due to flow behind the shock moving aroundthe cowl. In this scenario, MHD assist can be turnedon in the flow compression mode to force more flowinto the inlet and modify the pressure distribution tomove the bow shock to a shock-on-lip condition,eliminating spillage drag.

Scenario 2 is flight at higher than design Machnumber. Here the bow shock is ingested into theinlet, hi this scenario, MHD assist can be turned onin the expansion mode to modify the pressuredistribution and force the bow shock out of the inletand to a shock-on-lip condition.

Gross control over the inlet capture stream and cowlshock structure to be the greatest opportunity for theMHD inlet to advance hypersonic technology. Itpromises the ability to control the inlet design pointfor the aircraft and to modulate and/or augmentthrust.

The ability to control the hypersonic aircraft in suchsituations as the two scenarios discussed above, isundoubtedly a strong reason for initiating R/R&Dinto the MHD inlet concept. Furthermore, it isunderstood that combustion limitations exist forscramjets. These arise from the thermodynamic stateentering the combustor being at too low a pressure(~ '/2 Atm limit) and/or too high temperature. Theability of the MHD generator to control inlet massflow and to extract and dissipate power can providecontrol over the scramjet entrance thermodynamicstate (both pressure and temperature); thereby

providing the potential for increased scramjetoperating range.

An interesting mode of operation for the MHDcontrolled inlet is that in which an auxiliary powersupply placed in the power control circuit. The vectordiagrams of Figure 4 illustrate the electrical behaviorfor this type of operation. In terms of flow control,this is essentially the same as flow compression caseonly more forceful. Operation can be accomplishedby powering the generator at point 3 of the powerdiagram (see Figure 5). At this operating point, theFaraday current is neutralized, the Faraday currentvector is zero (Jy = 0) and thus the MHD drag thatoccurs from the MHD braking force (Jy x B) isneutralized. Faraday neutralized operation of theMHD inlet is a novel idea, however, it can requiresignificant power to force operation to the Jy equalszero point.

The concept of the "zero MHD drag" inlet ispredicated on driving the upstream portion of theinlet generator configuration with power producedfrom its downstream portion. This is similar to theAJAX energy by-pass notion

Pre-Compression SimulationsCFD analyses were made to allow study of theeffectiveness of the MHD inlet flow compression andexpansion processes. A geometric scenario was posedwith the general computational domain as illustratedin Figure 8. The domain considers flow over a flatplate simulating the forebody of the aircraft. The inletis situated down-stream of the leading edge asindicated. A four meter length in front of the cowllip is imposed as an MHD interaction region with

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ComputationalDomain

Region Subject toMHD Interaction

Flow

2 , . 3x~(m)

Figure 8. MHD Inlet Compression/Expansion Computational Domainmagnetic field taken as constant and extending threemeters upstream of the cowl lip. The cowl plate ismodeled as infinitely thin; introduced in this fashionto provide a visualization of the separation ofexternal and internal intake flow streams as MHDpre-compression forces the incoming flowdownward.

The height of the computational domain extends 0.5meters with the inlet height being 0.0625 meters andthe MHD interaction region extends 0.45 meters inheight. The inlet extends for one meter from the rightborder of the computational domain.

The numerical simulations consider the flow as two-dimensional, invicid, and a perfect gas, y=1.32. Theflowfield is calculated via Euler's equations withsource terms in the momentum and energy equationsto account for MHD push work and Joule heating.MHD interaction is introduced by imposing an axialcurrent across the domain that constitutes powering

of the inlet. In this situation, the MHD interactionparameters as normally defined, i.e.,

,or, Ip =

is not a true representation of the flow interaction.Rather, with current imposed the interaction over theregion is more appropriately,

Su=^pu2

that is independent of the plasma electrical pro-perties.

Contour plots of thermodynamic distributions aspredicted are shown in Figure 9. The color codingvaries somewhat but in general passes from light blue(free stream) to red as the intensity increases. Theconditions imposed for the calculation providesimulation of a Mach 10 incoming flow at

Figure 9. MHD Pre-Compression: Contour Plots of Flow Variables Upstream of the Inlet

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approximately 100,000 feet. At this flight conditionthis level of conductivity is only achievable with pre-ionization to produce a non-equilibrium plasma. Tointroduce the compression Lorentz force, an axialcurrent is imposed across the interaction region frominlet plane forward (to the left).

The methodology used to construct this simplifiedexample of MHD pre-compression is representativefor the MHD inlet. An axial current density, Jx, of0.45 Amps/cm2 is imposed which corresponds to asupply current of 2,000 Amps. The magneticinduction of the externally applied magnetic field isassumed to be constant inside the MHD interactionregion. With this orientation, the net Jx x B force isdirected normal to the wall, e.g., powering the inletwith current directed forward to produce compressionof the flow down against the forebody of the aircraft.

The general behavior evident in the contours is thatthe MHD interaction draws mass into the inlet.Quantitatively, the mass flow increase for this case isfour (4) times over that of a free flow condition (noMHD interaction). The density increase is on theorder of four times that of the free stream level andthe pressure increases from 1.0 kPa to 8.0 kPa, justinside the inlet. At the same tune, only a modestincrease in static temperature of 300K was indicated.

Figure 10 is a second an example of a 2D CFDsimulation of the preliminary analysis. This case ispre-compression of the incoming ionized airflow at aflight condition of Mach 10 and approximately100,000 ft (static pressure 1,000 Pa, statictemperature 250K). MHD interaction parameter is afactor of four over the previous case with a 1.0 Tesla

Flow

magnetic field, Hall parameter, or equal to 5.0, andelectrical conductivity is set at 50 S/m. Across thelength of the MHD interaction region, 5.5 kV isapplied, driving an axial current upstream with amean current density of 1.64. amp/cm2.

The upper figure shows contour plots of densityalong with flow streamlines. The strong draft of flowinto the inlet is observed by the streamlines turningtoward the inlet entrance and the compression.

Density increase at the inlet plane is on the order ofeight times that of free stream. The lower plotprovides contour of temperature with lines ofconstant current density. Large current eddies areseen to develop and extend out above the inlet cowl.These outer eddies are weak with the bulk of thecurrent issuing from the inlet passing in the upstreamdirection and being highly concentrated along theforebody. The integrated body forces were computedas; fx = -12 kN, and fy = -69 kN.

MHD Boundary Layer Control

The MHD flow control concept for the inlet containssub-elements that are of equal interest. The MHDinteraction processes within the inlet's sub-regions,e.g., the forebody and inlet internal walls boundarylayer, provides a mechanism to transform controlledelectric power directly into kinetic energy. This canbe an extremely effective MHD boundary layercontrol technique.

A general concept for a system for MHD boundarylayer (BL) interactions is illustrated in Figure 11. It isbased on the premise of flow of an ionized stream

-0.5

Flow

I I i I I I I I ( I I I I I I I I I I I I I I I i * i I I I I I I < I I 1 I I I I I I t I I I I I

Figure 10. MHD Pre-Compression: Contour Plots, Flow Variables Upstream of the Inlet Region

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(hypersonic flight air) over a surface with induced through the air plasma in alternating directions across

Electrodes

a. MHD BL Deceleration

Electrodes j x B

j x B

b. MHD BL Acceleration

Figure 11. MHD BL Interactions Over Airframe Surface - Illustration of Lorentz Force OrientationMHD interaction. Figure 11 shows the MHDinteraction in terms of vector definitions for thesystem. In this configuration, the surface has magnetsimbedded within it to produce the magnetic fieldorientations as noted. This positioning introduces analternating B-field vector as indicated.

Electrodes are imbedded into the surface and spacedlaterally. Electrodes can be either tied through anexternal load (generator mode) or powered from anexternal power supply (accelerator mode) to imposethe desired polarity between adjacent electrodes. Inoperation, electrical current passes between adjacentelectrodes with alternating reverse directions asindicated by the current vectors in Figure 11.Control over BL deceleration across a surface withMHD is by actively extracting electrical power fromthe induced EMF (U x B) to force current through theair plasma in alternating directions across the flowthat are locally orthogonal to an externally applied B-field. This produces a unidirectional Lorentz bodyforce (Jx x B) within the plasma in the direction asnoted in Figure 1 la. The body force is always in thedirection counter to the primary flow and produces adeceleration of the flow in the near wall region (BL).

Control over BL acceleration with MHD is byactively imposing an electric field to force current

Magnet(Turns)

Wedge

the flow that are locally orthogonal to an externallyapplied B-field. This produces a unidirectionalLorentz body force (J x B) within the plasma in theprimary flow direction as noted in Figure 12b. Thebody force is always in the same direction as the flow- producing an acceleration of the flow in the nearwall region (BL).

Note that the actions of the MHD interaction ascritiqued in Figure 1 1 will increase or decrease thekinetic energy of this layer and correspondingly giverise to a decrease or increase in the total pressurebehind the bow shock. This action over an extendedlength can result in significant modification of thepressure differential across the shock boundary - andthe shock wave strength changes accordingly.

The specific problem investigated by the authors wasthat of employing of MHD BL control to influenceseparation. The configuration studied was flow overa flat wedge simulating flow upstream of the inletover the vehicle forebody, as shown Figure 12. Withthe magnet configured as several adjacent, closelyspaced turns; the B-field distribution across thesurface alternates in direction and intensity asillustrated in the figure insert. In the presence of noexternally applied electric field, only induced eddycurrents will be present in the plasma boundary layer

4 4 4 t

___ I ___ I ___ [ /

turns

surface

Flow

Figure 12. Flow Over a Wedge MGD/MHD BL Control Configuration

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1.0

y(cm)

0

1.0

y(cm)

Figure 12. Behavior of Hypersonic Velocity and Thermal Boundary Layers Through MGD/MHD BL Interactionwhich produce alternating direction induced to thetransverse current, Jy. As a result, the Lorentz forces(Jy x B) opposing the incoming boundary layer flowwill vary in intensity but will always be directedcounter to the flow direction.

A typical result of one of case study is shown inFigure 14. This illustrates the development of thevelocity and thermal boundary layers over the wedgesubject to the influence of MHD interaction. For thecase shown, the B-field intensity was quite low (0.3T) and no external electric field is applied. Theattribute being sought was visualization of thephenomenological mechanisms at play as the bound-ary layer flow moves across the MHD interactioninfluence region and to determine the capability forthe MHD effect to trip (and potentially control)boundary layer separation.

The distortion of the velocity profile that developsdue to MHD interaction is non-typical to normal wallseparation. The high conductivity layer that exists inthe recovery region of the thermal boundary layerprovides a path through which the highest electriccurrent is forced. This region experiences anexaggerated Lorentz braking force in comparison tothe near wall and outer edge regions. This actiondistorts the boundary layer in the fas-hionsillustrated, i.e., the mid-layer being strongly retardedleads to velocity suppression in this layer whicheventually drives the velocity gradient to zero (du/dy= 0); the retardation of the flow in the mid-layergives rise to elevation in local static temperature andthe exagerated temperature bulge that develops in thethermal profiles.

Whereas; wall separation is normally identified whenthe velocity gradient at the wall passes through zero,in the MGD/MHD boundary layer this condition isfirst reached within the boundary layer region justabove the wall (recovery region). This behavior isunique to the MHD process. (One can hypothesizethat across the mid-layer region where du/dy isforced to zero, circulation will be induced and theboundary layer will tear itself apart, leading tocondition of total wall separation.)

In similar action, MGD/MHD interaction "squeezes"the boundary layer leading to a rapid growth of thedisplacement thickness and deflection of the externalflow. This is accompanied with a corresponding risein pressure to the point that the pressure gradient(dP/dx) quickly exceeds the index of incipientseparation (i.e., Pohlhausen parameter, A, exceeds12.)

A striking result of these first studies is the fact thatMGD/MHD interaction drives the boundary layer toseparation within only a 24 cm length. And, thisoccurs with low MHD interaction, i.e., B-field at 0.3T is within capability of permanent magnets. Thisfacet along with the rapid rise in surface pressure thatoccurs is an indication of the opportunity for use ofMGD/MHD as forebody and internal flow control forthe inlet. In addition, it implicates the additionalopportunity to use MHD BL control for hypersonicaircraft electric control of thrust, drag and lift; as wellas, thermal management, steering or aero-braking.

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MHD Control of Leading Edge Heat Transfer

The investigation on MHD BL control addressed inthe previous section was part of the efforts inscreening MHD designs and mechanisms for controlof convective loads on the forebody and internalwalls. The other area that has been cited of interest isMGD/MHD control over leading edge, stagnationpoint heat loads. It seems that this one of the mostpromising and effective MHD applications forhypersonics and the preliminary estimations maderecently tend to confirm such an expectations.

Further study conducted on the leading edge MHDeffects focused on analysis in support this concept.One configuration is shown in Figure 13 wherein arounded leading edge is configured with an internalmagnet turn to produce a magnetic field orientationin the circumferential direction with respect to theleading edge radius center. A representation of themagnetic field distribution is given on the insert plot.

With hypersonic, conducting flow approaching fromthe left, a current density, j, is imposed within theregion behind the bow shock in the transversedirection as depicted. This orientation produces aLorentz body force in the direction counter to theflow, forcing the bow shock away from the body.

The conditions posed were steady state hypersonicflow of a perfect gas at Mach 15 over and around aleading edge with radius of 2 cm. Constant values forviscosity and heat conductivity were used while thefull Navier-Stokes equations were numerically inte-grated. The conventional no-slip boundary conditionwas applied and a constant wall temperature of 600Kwas fixed. A closed form approximation for seededplasma conductivity as a function of pressure andtemperature was applied.

Calculations were made for varying magnetic fieldintensities of 2, 4, and 6 Tesla. No Hall effect wasconsidered in this first set of calculations but werecognize that Hall effect can be dominate in thissystem and probably will imply the need forreconfiguration in order to optimize the system. AnMOD situation is posed, i.e., no applied electric fieldwhich is tantamount to an open circuit MHDcondition.

The contours of Figures 14 show a non-linearcharacter in flow and shock structure redistributionwith magnetic field intensity. As the magnetic field isincreased, the stand off distance is increased by abouta factor of two for the cases investigated.

Leading EdgeBlunt Body

1cm

Figure 13. Leading Edge Configuration

Strong modification of the flow field is evident andthe character of the MHD interaction in different sub-regions of the control volume, i.e., stagnation pointregion and fringe regions around the body, isdependent upon the integral MHD interactionparameter. Since no Hall field is present in thesecalculated results, the actual flow field is defined bythe interaction parameter where the product B2L isthe most important (L being characteristic length).

The distribution of heat flux to the leading edgesurface is shown in Figure 15. This figure plotsdistribution on the rounded surface against axialdistance along the centerline.

Two observed effects in Figure 16 are consideredquite significant. First, the stagnation point heattransfer is reduced by nearly a factor of 2.0 betweenthe B-field cases. This behavior is consistent withmovement of the bow shock away from the body as

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<uX-axis

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(a) 2.0 Tesla (b) 4.0 Tesla (c) 6.0 TeslaPressure Distributions Around Round Leading Edge at Varying MHD Interaction

03X-axis

(a) 2.0 Tesla (b) 4.0 Tesla (c) 6.0 Tesla

Temperature Distributions Around Round Leading Edge at Varying MHD Interaction

Figure 14. Pressure and Temperature Distributions Around a Round Leading Edge at Varying MHD Interaction

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B-Field Intensify(Tesla)

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Figure 15. Heat Flux Distribution Around LeadingEdge with Variable MHD Interaction

interaction (B) is increased producing a shallowerthermal gradient on the nose of the body.

The second noticeable effect is that the heat flux inthe fringe region shows an opposite effect whereinheat transfer increases with increasing MHDinteraction. This behavior can be explained whenone considers that the reduction in stagnation pointheat transfer yields a higher temperature flow aroundthe body thereby increasing convective heat transferrate to the fringe region.

Consequently, our study shows that MHD control canreduce the "hot spot" leading edge by redistributionof the total heat load around the body. One visualizefrom the curves of Figure 15 that the overall heattransfer (integrated value of flux around the body) isfairly constant.

Boeing researchers in tests obtained experimentalverification of the reduced leading edge heat loadduring 1998 in the Calspan shock tunnel [12]. Thesetests utilized a blunt nose, body of revolution with amagnetic coil embedded beneath the surface of themodel. The experiment was an MOD configuration(i.e., no applied current). Experimental resultsshowed as much as a 50% reduction in heat transferto the body, comparing heat flux measurement whenthe magnet was on versus when it was off.

Concluding Remarks

This paper provides an overview of some novelconcepts that are under study exploring the use ofcontrolled MHD interactions to optimize flowfields

around hypersonic vehicles. This area of study is partof a broader, contemporary interest that exists in thefield of weakly ionized gas phenomena. Current workboth in this country and abroad is in pursuit ofinvestigation of means to utilize hypersonic plasmaflows to improve/advance in vehicle aerodynamicsand propulsion.

MHD flow control may well provide the means forenabling the next generation of hypersonic vehicles. Ithas application to both hypersonic cruisers and singlestage to orbit vehicles. Whereas, both vehicles seek tofly through the hypervelocity regime where ionizationof air exists (or can be induced) due to the flightdynamics.

In this publication we generically screened three areaswhere implementation of MHD systems can be used tocontrol flows for enabling advanced hypersonic flight.These encompassed inlet flow control, boundary layercontrol, and control over leading edge heat transfer.There are other MGD/MHD ideas under studyincluding on-board power production, shockmanipulation with sonic boom attenuation, directMHD propulsion and slip stream acceleration, MHDthrust vectoring, drag reduction, the MHD energy-bypass scramjet, and MHD aero-steering.

Highlights of some of the major elements/findings ofour paper are given in the following.

• Significant MHD interaction can be achieved inhypersonic flight for Mach numbers above about7. In the lower regime, either seeding of the flowwith alkali metal or energy addition to the flow toexcite ionization is required. Above about Mach12, natural ionization of air will provide sufficientconductivity to afford use of MHD controlconcepts.

• Utilization of MHD interaction to modify the flowstructure to the inlet of a hypersonic vehicle is anarea that needs to receive first attention. MHDcan produce a 'Virtual inlet" wherein directinteractions within the flow can provide inletcompression or expansion for control of the inletcapture stream and design point. A concept for aself-powered MHD inlet was tabled with thenovel "zero MHD drag" operating point noted.

• Active MHD control over boundary layers onvehicle surfaces and internal to the inlet isconsidered by the authors as an area for near termdevelopment. MHD boundary layer control canprovide means to reduce skin friction drag andwall heat transfer rate, and also retard/reduce flowseparation. Our first studies on surface effects

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indicated that a very striking means to induceflow separation can be achieved with MHD evenat moderate interaction level.

• Controlled MHD/MGD interaction imposed pro-perly in front of the leading edges on hypersonicvehicles can provide a means to reduce theprohibitive stagnation point heat transfer rates thatoccur in hypervelocity flight. Our preliminarystudies showed that the character of the heattransfer subject to MHD influence is amanifestation of two effects - the increase in theshock stand-off distance and the correspondingreduction in the thermal gradient in this regionleads to a significant reduction in the stagnationpoint heat transfer, and, the reduced leading edgestagnation point loss lends higher temperature tothe flow around the body which gives rise to theincreased convection in the fringe region.

References

;. Plenary Session 1-HYTASP-l, 9th InternationalSpace planes and Hypersonic Systems andTechnologies Conference, 1-5 Nov. 1999,Norfolk, Virginia, USA

2. V. A. Bityurin, V. A. Zeigarnik, A. L. Kuranov,On a Perspective of MHD Technology inAerospace Applications, AIAA-96-2355, 1996.

3. V. A. Bityurin and J. T. Lineberry, V.G.Potebnya,et al., Assessment of Hypersonic MHD Concepts,AIAA-97-2393, 1997.

4. D. Brichkin, A. Kuranov, E. Sheikin, MHDControl Technology for Hypersonic Vehicle, 2ndWeakly Ionized Gases Workshop, Apr. 27-30,1998,pp.239-261.

5. S. O. Macheret, M. N. Shneider, R. B. Miles,MHD Power Extraction from Cold HypersonicAir Flow with External lonization, Paper AIAA-99-4800.

6. T. Bogar, et al., Mach 10 Cruiser, 7th AIAAInternational Space Planes and HypersonicsSystems Technology Conference, Chattanooga,TN, Sept 1998.

7. A. B. Vatazhin, O. V. Gouskov, V. I. Kopchenov,Numerical Investigation of Hypersonic Inviscidand Viscous Flow Deceleration by MagneticField, Perspectives of MHD and PlasmaTechnologies in Aerospace Applications,Moscow, IVTAN, Mar. 24-25, 1999, pp. 13-18.

8. J. Cole, J. T. Lineberry, R. J. Litchford, and V. A.Bityurin, MHD Augmented HypersonicPropulsion System, Perspectives of MHD andPlasma Technologies in Aerospace Applications,Moscow, IVTAN, Mar. 24-25, 1999, pp.22-30.

9. V. A. Bityurin, J. T. Lineberry, A. N. Bocharov,MHD Aerospace Applications, InternationalConference on MHD Power Generation and HighTemperature Technology, Beijing, Oct. 12-15,1999, Vol.111.

10. A. B. Vatazhin, O. V. Gouskov, V. I. Kopchenov,The Investigation of Supersonic Flow Deceler-ation by Magnetic Field, , International Con-ference on MHD Power Generation and HighTemperature Technology., Beijing, Oct. 12-15,1999, Vol.III

11. R. J. Rosa, Magnetohvdrodvnamic EnergyConversion, McGraw-Hill, 1968

12. J. Mullen, Boeing Company, Oral Presentation,2nd Weakly Ionized Gases Workshop, Apr. 27-30, 1998

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