Drag Reduction
Base Bleeding
Introduction
In the world of aerodynamics, drag plays an important factor in designing new machines, from airplanes, cars, wind turbines, to military operations. Besides on using computers to approximate a possible solution , physical testing could be often performed as many unknowns can be introduced in the system thus changing the outcome of the experiment. Studying the effect on drag on different bodies plays a fundamental role in understanding the complex world of aerodynamics.
Since, there are many different possibilities to reduce drag on many objects, Base Bleeding plays a crucial part in the understanding of classical fluid mechanics that can be easily be applied to different fields of engineering. In this simple experiment ,we will focus on the classic aerodynamic shape used for different applications, “The airfoil”.
As we look forward in improving different types of machines by studying these effects it could potentially spread its use by improving our understanding . The following experiment is inspired after watching a base bleeding video of a Ferrari 488 GTB.
Project Description
Any aerodynamic/bluff body could produce a wake directly related to drag and the vortices it forms. Since the size of the wake and the boundary layer separation in an bluff body is directly related to the drag of the object. Base bleeding focuses on introducing flow to the base of the body in order to reduce the wake and/or intensity of the vortices it produces. We focused on a possibility of reducing drag by slightly modifying the area of the base of a control body.
In order to have a more stable model, we opted for an aerodynamic body produced by a symmetric airfoil. Symmetric airfoils are more stable and produce more uniform lateral and horizontal forces. Thus, minimizing the risk for vortex shedding, and early generation of vortices before the base of the body.
The airfoil used was on this test was NACA 0012
NACA 0012
The airfoil was then slight modified with a sharp edge to increase the wake and produce an induced boat tailing effect. Assuming if the body is not long enough the drag will significantly be larger and will produce a positive pressure gradient in the back of the base causing a suction effect and increasing drag in the body . As seen on the first video, If we redirect the flow to the base, we will then be able to reduce this pressure gradient or reduce the wake thus decreasing the drag.
The test was performed on 5 different bodies derived from the same airfoil.
Control Body
Modified Base area
Twisted Base area
Cutout Channel to base
Morel Body
Drag Calculation Method
Given that the wind tunnel used only provided lift/drag forces the following formulas were used to calculate the drag and lift coefficients.
The difference of velocities moving across the body causes the object to experience drag and lift. The shear force that the body will experience and the dynamic pressure across is integrated over the surface of the object to obtain lift and drag. In this case the force of lift and drag will be measured directly by the apparatus and the following formula will be modified using dynamic pressure and the area of the body to determine lift and drag coefficients.
Where ρ is the density of the fluid, CD is the coefficient of drag, CL is the coefficient of lift , V is the velocity of the fluid , and A is the area of the body which is
From this we can solve for the lift and drag coefficients
Sample Calculations
Control Body (TEST #1)
In the control body ,the airfoil was modified to have a sharper base tangential to the maximum thickness of the airfoil .
Average Cd = 0.076
MODIFIED BASE (TEST #2)
For the Second Test , the base was slightly changed to have a smooth transition from the regular cross section of the body to the base , keeping in mind that any sharp edges produced by changing the base could induce the separation of boundary layer and will introduce vortices in the system, increasing our drag.
Average Cd = 0.06
TWISTED BASE (TEST #3)
In the third test, we wanted to see what the effects would be , by twisting the already changed base of the body and what will this twisting do to the overall drag of the whole body.
Average Cd = 0.06
CUT-OUT CHANNEL (TEST #4)
In this test we wanted to affect drag by re-introducing the air flow from the middle part of the body to the base. The inlet area from the incoming flow is slightly larger than the area by the outlet made by the cutouts. If the area at the inlet is larger and the area at the outlet is smaller , we could then increase the velocity of the fluid and at the outlet close to the base where the cutout circles are seen. By this assumptions is possible that the re-introduced flow to the wake could have a much higher velocity thus having the possibility of minimizing drag.
Average Cd = 0.071
MOREL BODY (TEST #5)
For the last test, we consider a known practice of reducing drag “The morel body” which is to reduce the generation of concentrated vortices and eventually reduce drag. We choose 75 degrees of inclination based on the figure on the right, the Cd should be in between 0.2-0.3
Average Cd = 0.035
Final Results
After analyzing the data ,as expected the lowest drag coefficient was the one produced by Test #5 (morel body) reducing drag by 54.08 %.
The second lowest Test #3 (Twisted Base) reduced drag by 21.46 %
The modified base (Test #2) by 21.63% came out to be similar to the twisted base and demonstrated that the drag reduced is minimal and Test #2 could be chosen due to its simplicity.
An unexpected result came out to be cut-out base (Test #4) which was expected to reduce more drag but the test showed the opposite. If CFD was available a further test could have been performed in the cut-out channels to predict and improve the behavior of the air flow.
