Sho Kikkawa

Spectrum analysis of turbulence in the canine ascending aorta measured with a hot-film anemometer

Abstract We measured turbulence velocity in the canine ascending aorta using a hot-film anemometer. Blood flow velocity was measured at various points across the ascending aorta approximately 1.5–2 times the diameter downstream from the aortic valve. The turbulence spectrum was calculated and its characteristics were examined in connection with the mean Reynolds number and/or measuring positions. In the higher wave number range the values of the turbulence spectra were higher at larger mean Reynolds number. In the higher wave number range, the values of the turbulence spectra were higher at points closer to the centerline of the aorta, when the mean Reynolds number was relatively large. The patterns of the turbulence spectra at various points outside the boundary layer on the aortic wall were similar.

Spectral Broadening in Ultrasonic Doppler Flowmeters Due to Unsteady Flow

It is shown that time-varying velocity produces frequency modulation of the backscattered signal in ultrasonic Doppler flowmeters. Further, for blood flow in the ascending aorta, it is estimated that the spectrum of the backscattered signal would have a very wide bandwidth in the acceleration phase.

Methods of Measuring Blood Velocity

The hot-film anemometer is based on heat transfer from a small body placed in the fluid flow. This is a very small metal film which is sputtered or burned on a substrate or a mount. This small film is connected to a bridge circuit (Fig. 13.1) for the generation of heat by an electrical current. A servoamplifier feeds back the error voltage of the bridge circuit, so that the electrical resistance, which is a function of the film temperature, is kept constant. This type of electrical equipment is called a constant temperature anemometer (CTA). If heat loss from the film increases, due to the high velocity of the surrounding fluid, the film temperature falls and the electrical resistance decreases, which leads to the increase of error voltage of the bridge. In this situation, the feedback current or output voltage of the servoamplifier becomes a nonlinear function of the fluid velocity. The metal film should be covered by a thin layer of insulating material, usually quartz, which does not interfere significantly with the heat transfer.

Precursor of transition to turbulence in an oscillatory flow

The measurements of velocity on the turbulence occurring in a purely oscillatory pipe flow were performed by hot-wire anemometer for the Reynolds number Re (defined using the amplitude of cross-sectionally averaged velocity) of 30000 and for the value of the Stokes parameter λ=a/(v/w)1/2 (a=internal radius of tube, v=kinematic viscosity, w=angular frequency) of 8. The transition to turbulence occurrig in an oscillatory flow is evaluated in terms of turbulent intensity, skewness and flatness factors. The peak of skewness and flatness factors appears in front of the peak of turbulent intensity. Particularly, the rise of the flatness factor preceding the transition to turbulence signifies the appearance of an intermittent nature of turbulence and is characterized as a precursor of the transition to turbulence.

Spectrum analysis of turbulence in the canine ascending aorta

We measured turbulence velocity in the canine ascending aorta using a hot-film anemometer. Blood flow velocity was measured at various points across the ascending aorta approximately 1.5-2 times the diameter downstream from the aortic valve. The turbulence spectrum was calculated and its characteristics were examined in connection with the mean Reynolds number and/or measuring positions. In the higher wave number range the values of the turbulence spectra were higher at larger mean Reynolds number. In the higher wave number range, the values of the turbulence spectra were higher at points closer to the centerline of the aorta, when the mean Reynolds number was relatively large. The patterns of the turbulence spectra at various points outside the boundary layer on the aortic wall were similar.