Understanding Shock Waves in Aerospace Applications

the aircraft of today are no simple machines they not only push the boundaries of transportation military and defense they push the very boundaries of fluid mechanics listen to that that's the sound of a sonic boom an f-22 breaking the sound barrier and creating what we call a shock with understanding this phenomenon will allow you to unlock the mysteries of supersonic flight and truly appreciate the complexity of these machines this video will provide you with a conceptual framework of shockwaves as well as specific examples to help you study the different types of shocks it's one of the most intriguing properties of air something that drives the way we design all aircraft it all comes down to the special relation with the speed of sound picture an object moving through the air intuition might tell you that the air flows around the object like this moving out of the way and then coming back together there seems to know that the object is coming well this is because there are pressure waves that propagate forward from the leading edge of the object these slight increases and decreases in pressure cause the air to diverge you could think of these as sound waves because after all sound is just a disturbance and air pressure pressure waves are being transmitted what's really happening is molecules that interact with the surface reflect off the surface and then interact with other molecules that are sitting in the air in a subsonic case that's what's happening so the molecules can actually rebound off the body and then travel faster forward than the body does so the sound if you will travels faster the molecules travel faster than the body does so the flow in front of the body is being warned by these pressure waves so before the body gets there these pressure waves or sound waves we can think of them as are in front of the body and basically those sound waves are the underlying molecules moving faster than the vehicle itself these pressure waves are similar to a car's brake lights on the highway when one car slows down the one behind that will respond and so will the one behind that this passing of information is similar to how air molecules know to move out of the way but imagine this ball is now moving faster than the speed of sound roughly is 760 miles per hour the oncoming air is now quicker than these propagating pressure waves so there's no way to deliver the information of the approaching ball so there's an instantaneous deceleration of the air from supersonic to subsonic this is called a shock wave you can think about shock waves as many sound waves stacked on top of one another look at how the pressure waves from the front of the ball are all compressed down into one in a similar sense let's zoom out and look at this scenario here the pressure waves from the object propagate outward in all directions this is what it would look like if the object were not moving giving the object some forward velocity results in a way of distribution like this where the sound is compressed in the front and expanded in the back you might recognize this as the Doppler effect if you push it to the extreme past the sound barrier it looks like this notice how the constructive interference now lies along these lines that's the shock wave an observer standing here will therefore not hear anything but an observer here well you can see shockwaves come in a variety of flavors the type that formed in front of the ball is known as a bow shock or detached shock it is special in that it is not connected to the ball in any way it is also complicated to study since it very much depends on the geometry of the object but in general bow shocks will form only with blunt objects as the ball becomes more streamlined and eventually more pointed the shock wave will - until it hits a critical point where the bow shock becomes an attached oblique shock like this one this critical point depends on the speed of the flow and the angle of the cone aircraft designers want to avoid bow shocks at all costs since they provide a detrimental amount of drag a rounded front may work for subsonic airplanes like this one but supersonic aircraft like the f-22 have pointed tips to ensure oblique shocks oblique shocks are formed when the supersonic flow is rapidly redirected and they have little effect on drag because they do not actually slow down the air they actually speed it up the angle of the shock wave and the angle of the flow redirection are related to the Mach number by the theta beta Mach relation once again this angle is increased too much we revert to the bow shock case because the flow is unable to turn at that speed on aircraft like the sr-71 blackbird the engine inlets are large pointed cones to create oblique shocks and the aircraft travels at supersonic speeds these are carefully designed so that the shock waves fall just inside the entrance to the engine otherwise the drop in pressure would ruin the efficiency of the engine this cone can actually slide back and forth depending on the speed of the aircraft it's complicated machinery like this that utilizes the properties of shock waves so that covers oblique shocks and bow shocks but what if we explore the other extreme the flow is not turned at all this is what we call a normal shock since it forms perpendicular to the surface normal shocks occur when there is a change in the flow area and similar to bow shocks they change the flow from supersonic to subsonic similar to oblique shocks they are also attached to the body we can use the Mach jump relation to analyze how significant this deceleration really is normal shocks often form on the upper surface of wings because the flow speeds up enough locally to go supersonic since normal shocks also have drag high speed aircraft often have swept wings so that these normal shocks behave like oblique shocks and airspeed is maintained for a given amount of say lift that we have to generate in a transonic aircraft we'd like the shocks to be as weak as possible and and again that gets back to we want the losses that occur at a shock the entropy increase or the loss of efficiency to be as small as possible and the key factor there is really what's the Mach number normal to the shock wave if the Mach number normal to the shock wave is small that's going to help decrease the strength of that shock makes the losses less these three types of shock waves are very easy to confuse but by remembering how they affect aircraft design you will have a better understanding of their specific properties as you investigate shock waves further you will discover how they change not only the aerodynamics but also the thermodynamics of air and as we continue to push the limits of speed and efficiency we must be ever mindful of the effects of shock waves and how they drive our design decisions you

Channel: ESG PEV Spring 2015

Views: 98,054

Keywords: shock, waves, shockwaves, supersonic, subsonic, flight, aerospace, MIT, darmofal, aerodynamics, ESG

Id: j493HvCkMbM

Length: 8min 33sec (513 seconds)

Published: Wed May 06 2015