Cfd Analysis of the Propwash and Its Effects
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Transcript of Cfd Analysis of the Propwash and Its Effects
CFD ANALYSIS OF THE PROPWASH
AND ITS EFFECTS ON THE
AERODYNAMIC EFFICENCY AND
STABILITY OF THE MAVs
RAJASEKAR P
REJISH J
SATHEES KUMAR B
SILVA FRANKLIN S
Live project in – Experimental Aerodynamics Division “NATIONAL AEROSPACE LABORATORIES”, Bangalore. Under the guidance of Dr. Ramesh and Mr. Hemanth Sharma
OUTLAY OF PRESENTATION
INTRODUCTION TO MAV
PROPELLER EFFECTS
• PROP WASH
• P- FACTOR
• TORQUE
• GYROSCOPIC EFFECTS
PROPWASH EFFECTS ON AERODYNAMIC
EFFICIENCY AND STABILITY
LITERATURE SURVEY
BASICS OF CFD
INTRODUCTION TO MAV
• Micro Air Vehicle or Micro Aerial Vehicle is a class of Unmanned
Aerial vehicle that has size restrictions and may be autonomous
• Modern MAV can be as small as 15cm.
• The range of Reynolds number at which MAVs fly is similar to that
of an insect or bird (103 - 105)
CLASSIFICATIONS OF MAV
MAV
BASED ON WING
ROTARY WING
FIXED WING
FLAPPED WING
BASED ON PROPELLERS
PUSHER TYPE
TRACTOR TYPE
PROPELLER EFFECTS
Prop wash: p-factor:
Gyroscopic Precession: Torque effects:
PROPELLER EFFECTS
Spiral prop wash:
The turning propellers sends a spiraling column of air rearward that strikes the left side of tail and tries to push the tail to the right and yaw the nose towards the left is know as Prop Wash
The Prop does not throw the prop wash straight back there is some drag on the prop, and that tends to make the wash behind it come off in a spiral fashion and the problem comes when that spiral flow meets the rudder. If the rudder is mounted high, the plane will turn (yaw) left because only the top part of the spiral hits it.
NEUTRALIZING PROP WASH
Flight path due to prop wash
Right thrust
Neutralized flight path
ASYMMETRIC PROPELLER THRUST ( P- factor)
P- FACTOR
When a plane is pitched up into a climb or loop ,the angle of attack is made greater than the actual flight path At positive angle of attack the propeller blades on the right side of the plane bites more air than the blade on left side, resulting in more thrust on the right side, trying to push the nose left side
TORQUE
Since all propellers turn to the right, that means there is a force trying to twist (roll) the airplane to the left. Note that this force is about the ROLL axis- the torque forces do not by themselves turn or yaw the plane as do the other effects
Prop wash effects on MAV
This figure shows the contour of velocity in X-direction for propeller design condition (8000rpm and 12m/s inlet velocity). As air passes through the propeller wakes are formed behind the propeller due to the rotation
The axial velocity created by the propeller continues to be higher than the free stream velocity even up to a distance of 5 diameters from the propeller plane in the axial direction. This difference is indicates that, axial velocity is 5% higher than the free stream velocity at the end of downstream.
Variation of axial velocity
This diagrams shows the velocity vector behind the propeller for design condition
Velocity vector diagram:
Prop wash effects on MAV Aerodynamics
This figure shows the separation bubble which formed on the upper wing surface without the propeller effects
Without propeller:
This shows the bubble has been reduced due to the effects of propeller flow and the bubble becomes asymmetric this triggered the side force.
With propeller:
Flow over an MAV at 17 ° angle of attack
This shows the pressure distribution and streamlines for the stationary, elliptical airfoil • The dark region under the leading edge shows a region of
high pressure• The lighter area in the wake region shows a region of lower
pressure.• The streamlines plotted in the figure shows the beginnings of
separation in the wake region.
Flow over an MAV at 30° angle of attack
This shows a screen shot of a stationary, elliptical airfoil with a thirty degree angle of attack. Re =1000.• The model shows dark pockets of low pressure in the wake
region that get progressively lighter as the distance from the trailing edge of the airfoil increases.
• These regions of low pressure creates vortices that detached from the airfoil and flowed through the wake region.
Future work:
1. By using CFD we are going to analysis the prop wash effect for the MAV with tractor type propeller at the design condition by comparing with experimental results.
2. For the various angle of attack, we are going to calculate
• Aerodynamic efficiency (L/D ratio)• Coefficient of drag CD
• Pitching moment coefficient CM
• Side force coefficient CF
BASIC ASPECTS IN NUMERICS OF CFD
Discretization:
It is the process by which a closed form mathematical
expressions, such as a function or a differential or integral equation
involving functions, all of which are viewed as having an infinite
continuum of values throughout some domain, is approximated by
analogous expressions which prescribe values at only a finite number
of discrete points or volumes in the domain.
Discrete grid points:
Analytical solutions of PDE gives the variation of the dependent variable continuously throughout the domain
Numerical solutions can give answers at only discrete points in the domain, called grid points.
Structured grids :
The grid points are placed in a regular intervals (i.e.) if ∆x and ∆y are constant along x and y direction
Unstructured grids:
The grid points are placed in irregular fashion (i.e.) if ∆x and ∆y are not constant along x and y direction
Note : ∆x does not have to equal ∆y
Discretization techniques
Finite difference
Finite volume
Finite element
Finite Difference Methods:• The PDE is replaced with algebraic equations which
prescribe values at only a finite number of discrete points
• Difficult for complex geometries
Finite Volume Methods:• Convert the integral equations to a system of
algebraic equations.• Any flow domains can be solved.
Finite Element Methods:• Convert partial differential equation (PDE) as
well as integral equations to a system of algebraic equations.
FINITE DIFFERENCE METHOD
If u denotes the x component of velocity at points (i ,j ), then the velocity u at point (i+1,j) can be expressed in terms of Taylor series expanded about point (i, j) is given by
Taylors series:
Example problem:
The function is given by f(x) = sin 2πx
At x = 0.2; f(x) = 0.9511Corresponds to point 1 At x = 0.22; f(x) = 0.9823 Corresponds to point 2
• Now by using just first term on the right hand side Taylor series expansion
f(0.22) ≈ f(0.2) = 0.9511This corresponds to point 3 in the fig
The percentage error in this estimate is [(0.9823 – 0.9511)/0.9823]*100= 3.176% • By using two terms in the series
f(0.22) ≈ f(0.2) + 2π cos [2π(0.2)](0.02) ≈0.9511+0.388 = 0.9899This point corresponds to 4 in the fig
The error is 0.775%
The finite difference representations of derivatives is given by
Truncation error: This error tells us what is being neglected in this approximations
First order forward difference
First order backward difference
Second order central difference
Second order central second differences
Second order central difference for the mixed derivative
Explicit and Implicit Methods Explicit Method :
Flow properties at previous step are used to calculate new values at current time step
Implicit Method: Flow properties at previous and current time step are
used to calculate current time step