[DeTomaso] Other Aero Issues

michael frazier red3644 at hotmail.com
Mon May 19 15:25:58 EDT 2008


That's what i've been saying all along. :)  Dimples can be shiny too if that's what you have to have.  I need to do some serious tuft
testing to determine where actual flow revesal occurs.  I'm planning to cover the body with paper mache' next year and would feel better
knowing what overlapping pattern to lay the paper down in.  I wouldn't like leaving large sections of flour pasted newspaper on the course,
though that's not as bad as scoops, oil, coolant or flaming wreckage like Dennis does.  Thanks.
Michael> Date: Mon, 19 May 2008 10:47:44 -0500> From: daniel.c.jones2 at gmail.com> To: detomaso at realbig.com> Subject: Re: [DeTomaso] Other Aero Issues> > The dimples on a golf ball decrease drag by inducing a turbulent boundary> layer to delay the drag increase associated with separated flow. The least> drag is achieved when the boundary layer is laminar and the flow stays> attached to the surface contour. In practice, this only occurs passively> for slender shapes, like air foils) and then only sometimes. For most> practical shapes, laminar flow boundary layers will separate from the> surface. This separation causes a large drag increase. In cases like> that, a turbulent boundary layer stays attached to the surface longer,> resulting in less overall drag. It takes a fair bit of math to prove> this but it comes down to the fact that a turbulent boundary layer is> more energetic than a laminar one and requires a larger adverse pressure> gradient to detach. The location along the body at which the flow> transitions from laminar to turbulent determines the critical Reynolds> number. Below this number, the flow is laminar, above it's turbulent.> The Reynolds number is a linear function of velocity. The faster you> go, the farther forward the transition point moves. You don't have to> rely on high speeds to cause the boundary layer to "trip" from laminar> to turbulent. Small disturbances in the flow path can do the same thing.> That's why golf balls have dimples. A better approach for a Pantera> would boundary layer trip strips or vortex generators to do the same> thing. However it may be unnecesary. If the flow is already fully> turbulent ahead of the point at which the flow deaches, introducing> additional turbulence will not have a beneficial effect. Also, the> body needs to have a gradual slope for even a turbulent boundary layer> to stay attached. Flow will detach at the pronounced step at the sugar> scoop.> > A couple of guys here at work (Boeing), tufted an '87 Mustang LX hatchback> from the center of the roof to the taillights. They were trying to use> vortex generators to to induce turbulence to trip the boundary layer,> hoping to delay the flow dettachment at the hatch so they could get cleaner> air over the wing. Their vortex generators were based on aircraft designs> and they used a hang glider airspeed indicator on a pole to measure the> boundary layer thickness across the roof. They made the vortex generators> two inches tall to be conservative (the boundary layer was approximately> one inch thick and a rule of thumb is to make the generators 1.5 time the> boundary layer thickness). They didn't see an improvement in coast down> times, but the tufts did appear a little better behaved with the vortex> generators. They believe the turn at the back of the roof may be too sharp> to permit attached flow. They also noted that much of the clean flow to> the wing appeared to be coming from around the sides of the car.> > More on boundary layers follows...> > The profile drag of an object can be spilt into two components:> > Cd = Cdf + Cdp> > where> > Cd = profile drag coefficient> Cdp = pressure drag coefficient due to flow separation> Cdf = skin friction drag coefficient due to surface roughness> in the presence of laminar/turbulent flow> > The drag which comprises the Cdf component is caused by the shear stress> induced when air molecules collide with the surface of a body. A smooth> surface will have a low Cdf. Also, the Cdf is lower for laminar flow and> higher for turbulent flow. Cdp, on the other hand, is caused by the> fore-and-aft pressure differential created by flow separation. Usually,> Cdp is lower for turbulent flow and higher for laminar flow. In many cases,> inducing turbulence will dramatically decrease the pressure drag component,> decreasing the overall drag. Airplanes use this trick all the time.> > Back in the 19th century, when scientists were just beginning to study the> field of aerodynamics, an interesting observation was made with respect to> the drag of a cylinder. Since a cylinder is symmetric front-to-back (and> top-to-bottom), their early theories predicted it should have no drag (or> lift). If you plot the (theoretical) pressure distribution along the> surface of the cylinder (remembering that pressure acts perpendicular to a> surface) and decompose it into horizontal (drag) and vertical (lift)> components, you'll find that the pressure on the front face of the cylinder> (from -90 to +90 degrees) and the pressure on the rear face ( from +90 to> +270 degrees) are equal in magnitude but opposite in direction, exactly> cancelling each other out. Therefore, there should be no drag (or lift).> > However, if you actually measure the pressure distribution, you'll find> there are considerably lower pressures on the rear face, resulting in> considerable drag. This difference between predicted and observed drag> over a cylinder was particularly bothersome to early aerodynamicists who> termed the phenomenon d'Alembert's paradox. The problem was due to the> fact that the original analysis did not include the effects of skin> friction at the surface of the cylinder. When air flow comes in contact> with a surface, the flow adheres to the surface, altering its dynamics.> Conceptually, aerodynamicists split airflow up into two separate regions,> a region close to the surface where skin friction is important (termed the> boundary layer), and the area outside the boundary layer which is treated> as frictionless. The boundary layer can be further characterized as> either laminar or turbulent. Under laminar conditions, the flow moves> smoothly and follows the general contours of the body. Under turbulent> conditions, the flow becomes chaotic and random.> > It turns out that a cylinder is a very high drag shape. At the speeds> we're talking about, a cylinder has a drag Cd of approximately 0.4. By> comparison, an infinite flat plate would have a Cd of 1.0. Note that> this is not a theoretical limit. A rectangular beam will exhibit flow> separation at each corner and can have a Cd in the range of 2.0. An> efficient shape like an airfoil (that is aligned with the airflow, i.e.> is at 0 degrees angle of attack) may have a Cd of 0.005 to 0.01. Think> about what this means. An airfoil that is 40 to 80 inches tall may have> approximately the same drag as a 1 inch diameter cylinder.> > Luckily, there are easy ways of reducing a cylinder's drag. Another thing> the early aerodynamicists noticed was that as you increased the speed of> the air flowing over a cylinder, eventually there was a drastic decrease in> drag. The reason lies in different effects laminar and turbulent boundary> layers have on flow separation. Laminar boundary layers separate (detach> from the body) much more easily than turbulent ones. In the case of the> cylinder, when the flow is laminar, the boundary layer separates earlier,> resulting in flow that is totally separated from the rear face and a large> wake. As the air flow speed is increased, the transition from laminar to> turbulent takes place on the front face. The turbulent bundary layer stays> attached longer so the separation point moves rearward, resulting in a> smaller wake and lower drag. For a cylinder, laminar flow separation may> occur at 82 degrees (with the leading edge of the cylinder at 0 degrees)> and yield a Cd=1.2. With a turbulent boundary layer, flow can stay attached> to around 120 degrees, resulting in a decrease in drag of Cd=0.3. The same> effect occurs for similarly sized sphere which can have a Cd=0.5 under> laminar conditions and a Cd=0.2 under turbulent conditions.> > The location along the body at which the flow transitions from laminar to> turbulent determines the critical Reynolds number. Below this number, the> flow is laminar, above it's turbulent. The Reynolds number is defined as:> > Re_x = (Rho * V * X)/Mu> > where:> > Re_x = Reynolds number at location x (a dimensionless quantity)> Rho = freestream air density> V = freestream flow velocity> x = distance from the leading edge> Mu = freestream viscosity, a physical property of the gas (or liquid)> involved, varies with temperature, at standard conditions mu is> approximately 3.7373x10E-07 slug/(ft*sec) for air.> > Since the Reynolds number varies linearly with the location along the body> and with velocity, the faster you go, the farther forward the transition> point moves. At cruising speed on a typical jet airliner, only a small region> near the leading edge may be laminar. Slow speed gliders with very slender> (but still with rounded, blunt, leading edges) may maintain laminar flow over> most of the wing surface but this is not the case for most practical aircraft.> Note that glider wings are typically designed with very short chord lengths> (x distances) to help promote laminar flow. Laminar flow is desirable when> there is no pressure separation.> > You don't have to rely on high speeds to cause the bondary layer to "trip"> from laminar to turbulent. Small disturbances in the flow path can do the> same thing. A golf ball is a classic example. The dimples on a golf ball> are designed to promote turbulence and thus reduce drag on the ball in> flight. If a golf ball were smooth like a ping pong ball, it would have> much more drag. If you look closely, you'll notice that some Indy and F1> helmets have a boundary layer trip strip to reduce buffeting. It seems odd> but promoting turbulence can reduce buffeting by producing a smaller wake.> > Another consequence of skin friction on a cylinder is that you can generate> substantial lift with a spinning cylinder. By spinning a cylinder you can> speed up the flow over the top and slow down flow under the bottom, resulting> in a lift producing pressure differential. I think this phenomenon is known> as the Magnus effect. BTW, the spinning tires on F1 and Indy cars are *huge*> sources of drag.> > Dan Jones> _______________________________________________> > Detomaso Forum Managed by POCA> > Archive Search Engine Now Available at http://www.realbig.com/detomaso/> > DeTomaso mailing list> DeTomaso at list.realbig.com> http://list.realbig.com/mailman/listinfo/detomaso
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