Acoustics Shock Vibration Signal Processing September 2011 Newsletter Wake Turbulence by Tom Irvine Figure 1. Wingtip Vortices Introduction Airflow over an aircraft’s wings generates of pair of counter-rotating vortices, as shown in Figure 1. The vortices are hazards to other aircraft following behind in the wake, or crossing through the wake. A particular concern is that the trailing aircraft may experience severe displacement and acceleration, causing loss of control, structural damage, or even a fatal crash. Flow Mechanism The air pressure above the wing is less than the air pressure below the wing during flight. This is necessary for lift. The high-pressure air below the wing flows upward to the low pressure region. The fuselage blocks this flow, so the path of least resistance is toward the wingtips. Meanwhile air curls up over the wing tips and then flows from the top of the wing downward, creating down wash, as shown in Figure 2. The two flow effects combine at the wingtip and create a fast spinning vortex similar to horizontal tornadoes, trailing the aircraft. Characteristics The main characteristics of aircraft wake vortices are: Sink rate: 300 to 500 feet/minute Stabilization at 500 to 900 feet under the aircraft at the origin of the vortices Lateral movement at 5 knots when reaching the ground Life span: 30 seconds to a few minutes depending on the aircraft’s size, weight, speed and wing shape, as well as the wind conditions
NASA Aerodynamicist Craig Streett wrote: Trailing vortices do in fact sink by their mutual interaction, but they also move together, until they become unstable and pinch off into connected rings - the so-called Crow instability. There are literally thousands of pictures and studies documenting this behavior. Only when the tip vortices are in ground effect, when they are interacting with their below-ground images, do they travel away from each other. Also, you needn't appeal to a heuristic argument to explain the existence of trailing vortices. Inviscid vortex dynamics requires that a vortex cannot simply end in free space, and so the bound vortex responsible for the lift of the wing must go somewhere at the wing tip; this is the so-called Lighthill horseshoe vortex system, which also explains the cause of induced drag. Finally, while the popular press calls encounters under the generic term of "turbulence", they are classified formally in flight sciences as "gust" encounters, to distinguish from true atmospheric turbulence resulting from meteorological mechanisms. Atmospheric Turbulence By Tom Irvine Figure 1. Jet Streams Jet streams flow from west to east in the upper portion of the troposphere. They are caused by the temperature difference between the warm equator and cold poles. High pressure air from the warm tropics flows toward the low pressure at the cold poles, but the Coriolis force from the Earth's spin deflects these winds to the east so that they make an eastward circle around the globe. Introduction are numerous sources of atmospheric turbulence, including mountain waves, instabilities, thunderstorms, jet stream, as well as thermal updrafts and downdrafts. Clear Air Turbulence The turbulence may be clear air turbulence (CAT) if there are no clouds or other visible signs. This turbulence can cause "air pockets." An air pocket is a downward air current that causes an aircraft to undergo a sudden decrease in altitude. Jet Stream The main jet streams are located near the tropopause, the transition between the troposphere (where temperature decreases with altitude) and the stratosphere (where temperature increases with altitude). Jet streams are caused by a combination of the rotation about its axis and atmospheric heating. The Jet Stream is formed as a result of the temperature gradient between the cold polar air mass and the warmer sub-tropical air mass.
diverted to Auckland, New Zealand so that the injured could receive medical treatment. The aircraft was not damaged. According to the captain the airplane was flying over a flat broken cloud layer with a smooth ride prior to the turbulence event. He reported, in part: "Passing 25 degrees south at FL310 we noticed that the cloud tops were gradually rising. Radar showed very little - a few green returns off to the right. We were in an area of no forecasted turbulence or cumulonimbus buildups. I turned on the seatbelt sign and made a passenger announcement." The Digital Flight Data Recorder (DFDR) data showed that the airplane experienced a series of oscillating vertical and lateral accelerations that lasted almost 2 minutes. The vertical accelerations (expressed in units of gravity or G’s, 1.0 is normal) ranged from a low of +0.31 to a high of +1.7. The lateral accelerations (a value of zero is normal) ranged from 0.119 left to 0.115 right. The National Weather Service Significant Weather Forecast Chart valid for the flight showed a 120 knot jet stream and the possibility of occasional moderate or lesser clear air turbulence south of the accident location and an area of isolated cumulonimbus clouds with tops to 40,000 feet to the north. Source: National Transportation Safety Board accident database system (ADMS2000), last updated Jan 1, 2010. NOAA Hurricane Hunter By Tom Irvine Figure 1. Lockheed WP-3D Orion, N42RF Introduction NOAA has two WP-3D Orion turboprop aircraft, designated N42RF and N43RF. The aircraft participate in a wide variety of national and international meteorological, oceanographic and environmental research programs in addition to their widely known use in hurricane research and reconnaissance. The aircraft are equipped with an array of scientific instrumentation, radars and recording systems for both in-situ and remote sensing measurements of the atmosphere, the earth and its environment. Hurricane Felix Hurricane Felix was a destructive Category 5 hurricane on the Saffir-Simpson hurricane scale that struck Central America in 2007 formed from a tropical wave on August 31. It made landfall just south of the border between Nicaragua and Honduras On September 4, in a region historically known as the Mosquito Coast with 160 mph (260 km/h) winds. Figure 2. Hurricane Felix during rapid intensification