1. Control of Boundary Layer Structure for Low Re Blades
P M V Subbarao
Professor
Mechanical Engineering Department
Creation of Geometry to Achieve Perfect Boundary Layers……

2. Structure of Boundary Layer Suction Surface of an Efficient Airfoil

3. The Flow on Suction Side of A Blade with Perfect Airfoil Shape
The flow on the suction side of an well behaving blade is expected to reach transition on the airfoil surface without the mechanism of a laminar separation bubble.
Prior to transition, the drag increases slowly, while downstream of transition the growth is more rapid and consistent with a turbulent boundary layer.
Such a flow is characterized by
the position of laminar–turbulent transition,
transition length and
diffusion rate.

4. The Perfect Blade Profile : Distribution of Local Velocity
Ve,max
Ve,tail

5. Lift Vs Drag
To reduce skin friction loss on the profile, the fraction of laminar surface length on the suction side has to be extended.
The friction loss varies with (velocity)2 for a laminar boundary layer.
The friction loss varies with(velocity)3 for turbulent boundary layer.
For a given anlge of attack and chord, increase in the laminar length implies a shift in loading towards trailing edge.

6. Flow deceleration on the suction side
Improvement of transitional behaviour and growth of boundary layer is important.
Arrange the curvature distribution on the suction side to create a pronounced local adverse pressure gradient but a very mild diffusion.
This configuration enforces a well defined transition location and the ensuing turbulent boundary layer is relaxed, through very mild diffusion.
These measures improve the performance level and transitional behaviour.

7. Local Tuning of Blade Shape
Compute the surface potential function, 
The flow potential is perturbed and hence the local flow is influenced by the airfoil shape/blade shape.

8. Control of Flow Deceleration on Suction Side
Ve,max
Ve,tail v2

9. Control of Flow Deceleration on Pressure Side
For a given profile, a certain amount of initial deceleration on the pressure side is helpful.
This allows reduction in the maximum velocity level on the suction side, thereby reducing the overall diffusion on the latter.
Local changes in geometry in terms of both profile contour change and the corresponding velocity distribution is helpful in this regard.

10. Control of Flow Deceleration on Pressure Side
Note that, owing to the lower dynamic head on the pressure side, substantial geometry change is required to produce a measurable effect on the suction side.
Ve,max
Ve,tail

11. Rules to Contain Profile Losses : Tuning of Suction side Profile

12. Rules to Contain Profile Losses : Tuning of Pressure side Profile

13. Surface Pressure Distributions
The aerodynamic performance of airfoil sections can be studied most easily by reference to the distribution of pressure over the airfoil.
This distribution is usually expressed in terms of the pressure coefficient:
Cp is the difference between local static pressure and freestream static pressure, non-dimensionalized by the freestream dynamic pressure.
Invoke Bernoulli's equation:

14. Local Velocity over an Airfoil
For incompressible flow, The corresponding pressure coefficient can then be found using V

15. Airfoil Pressure Distributions

16. Airfoil Pressures and Performance

17. Angle of Attack Vs Pressure distribution : Perfect Blade
Increasing Angle of Attack

18. Angle of Attack Vs Pressure distribution : Real Blade

19. Front Loaded Vs Aft Loaded Blades

20. Aft Loaded Blades with High Diffusion

21. Generation of Separation Bubble
Higher diffusion rates are responsible for creation of separation bubble.
Low Reynolds number/ laminar flow tend to create long separation bubble.
This action promote aft-loading to a significantly higher degree.
In order to avoid substantial trailing edge loss, it is important to ensure that flow reattachment is completed before reaching the trailing edge.

22. Adverse Pressure Gradient : Boundary Layer Separation
Laminar separation bubbles are caused by the inability of the boundary layer flow to make an early transition to turbulent flow.
Instead, the laminar flow separates before transition.
Flow separation creates a free shear flow.
Transition readily occurs in the free shear layer, and turbulent flow reattaches to the airfoil surface downstream of transition.
A Laminar separation bubble (LSB) is formed.

23. Anatomy of BL with LSB

24. Effect of LSB on Boundary Layer Edge velocity
The drag increment due to a laminar separation bubble is proportional to the drop in the edge velocity. Ve
Small LSB
Large LSB
No Separation

25. Problems with Blade with Large Adverse Pressure Gradient
Separation Bubbles & Reattachment - -

26. The Size of LSB
Airfoils at low Reynolds numbers possess large region of the laminar separation bubble on suction side .
They experience the resulting pressure drag over the suction surface and the relatively high drag.
The existence of a laminar separation bubble and its extents can be deduced by examining and modification of suction side geometry of airfoil.
The drag contribution owning to a bubble can be approximated by considering the surface pressure distribution.

27. Control of Reattachment Point
A lower diffusion rate can readily be achieved by shifting the profile loading to the front part of the profile (front-loaded profile, FLP).
This has very interesting features of a low diffusion rate on the suction side of such profiles.
However, the low level of adverse pressure gradient causes the transition point to be unstable (higher transition length).
This length is more sensitive to inlet turbulence variations.
Theoretical predictions based on 2D blade boundary layer calculations and on wind tunnel measurements have clearly confirmed that a lower profile loss can be reached with FLPs at the design point.

28. Angle of Attack Vs Pressure distribution : Real Blade

29. Design Philosophy
The performance of low Reynolds number airfoils is strongly dependent on the location of transition.
This sets the length of the laminar separation bubble and consequently the magnitude of the drag rise attributable to the bubble.
Controlling transition is a key step towards mitigating the adverse effects of laminar separation bubbles on low Reynolds number airfoils.
One common approach to controlling transition, is to employ an instability region in the pressure gradient or, as it is commonly called, a transition ramp.