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9.10 Turbulent Flow Studies 247

functional groups to the silica surface to render the surface strongly hydrophobic.
The latter is necessary as the refractive index needs to be controlled when the fiber is
immersed in liquids. A dual optical probe for local volume fraction, drop velocity,
and drop size measurement in a kerosene-water two-phase flow was published by
Hamad et al. [116]. A method for nonintrusive measurement of velocity and slug
length in multiphase flow in glass capillaries of 1- or 2-mm inner diameter was pre-
sented by Wolffenbuttel et al. [117]. (Slug here means an amount of fluid and not
the animals that eat, uninvited, the salad leaves of hobby gardeners.) A combination
of an impedance meter and two infrared sensors is used to distinguish between air,
water, and decane [Figure 9.36(c)].

9.10 Turbulent Flow Studies

An area where MEMS sensors have considerably broadened the field of study is
fluid dynamics. A typical MEMS sensor is at least one order of magnitude smaller
than conventional sensors used to measure instantaneous flow quantities such as
pressure and velocity [118]. The micromachined sensors are able to resolve all rele-
vant scales, even in high Reynolds number turbulent flows. Due to their small size,
the inertial mass and the thermal capacity are reduced. Thus, they can be used for
the study of turbulent flows, where a high-frequency response and a fine spatial
resolution are essential. The smallest scales of eddies in turbulent flow are in the
order of 100 µm [64]. Arrays of microsensors could make it possible to achieve
complete information on the effective small-scale coherent structures in turbulent
wall-bounded flows. Applications of turbulent flow study include the optimization
of wing sections of aircraft, the minimization of noise generation of vehicles, or mix-
ing enhancement for fluids.
The goal of measuring turbulent flows is to resolve both the largest and smallest
eddies that occur in the flow. In order to obtain meaningful results, both wall pres-
sure and wall shear stress need to be measured [118]. The wall shear stress is the
friction force that a flow exerts on the surface of an object.
The wall pressure can be measured with the sensors described in Chapter 6.
Löfdahl et al. [118] recommends that the pressure sensor needs to have a mem-
2
2
brane size between 100 ×100 µm and 300 × 300 µm , it needs to have a high sen-
sitivity of ±10 Pa, and the frequency characteristic should be in the range of 10 Hz
to 10 kHz. The wall shear stress is a parameter of small magnitude. For example, a
submarine cruising at 30 km/h has an estimated value of the shear stress of 40 Pa;
an aircraft flying at 420 km/h, 2 Pa; and a car moving at 100 km/h, 1 Pa [118].
Therefore, the sensitivity of shear stress sensors needs to be very high. For wall
shear stress sensors, there are direct and indirect measurement methods. For
MEMS devices, the direct measurement method is the floating element method.
Here, the sensor needs to have an element movable in the plane of the wall, which is
laterally displaced by the tangential viscous force. The movement can be measured
using resistive, capacitive, or optical detection principles. It is an important
requirement that the sensor is mounted flush to the wall. Misalignment and gaps
around the sensing element, needed to allow small movements, are sources of error.
For conventional “macro” sensors, effects that could cause measurement errors are
pressure gradients, heat transfer, suction/blowing, gravity, and acceleration. With