Bio-mimetic Drag Reduction – Part 1: Sensing

Fish have a remarkable ability to sense the flow conditions around their bodies and subsequently manipulate their swimming behaviour to achieve efficient locomotion [1 – 2]. It has been observed that dolphins and sharks use a network of mechanosensors on their skin to create a spatial image of the flow around them and use this flow information to minimise drag by active skin vibrations [1, 3 – 4], mucus excretion [1, 4 – 5], undulation frequency optimisation [4, 6], vortex generation [1 – 2, 3, 5], passive bristling of scales and external riblet profiles [3, 7 – 8]. This will be the first part of a three-piece post on drag reduction techniques inspired by fish.

This post will focus on flow sensing, while in future posts I will introduce the different morphing mechanism that fish, dolphins and sharks exhibit and how bio-mimetic inspiration could be utilised to reduce drag on future aircraft. For the purpose of this post I will refer to the family of undulating fish, dolphins and sharks as “fishes” even though they may not be classified in the same biological family.

Water currents in aquatic environments are of equal importance to life as gravity and light to us on shore. Plankton-feeding fish sense the flow direction of surrounding currents to orient themselves facing into the flow and intercept drifting food items. Furthermore, fishes hold position in low velocity flows that provide and abundance of invertebrate drift around there bodies. When fishes glide they create a dipole flow field around their bodies that has been shown to serve as a means of communication during schooling. In the case of the blind Mexican cave fish, Astyanax fasciatus mexicanus, the reflections of the disturbance waves created by the swimming fish is also used to create a spatial image of the environment around them. It is therefore no wonder that fishes are well equipped anatomically to respond and take advantage of the flow fields around them.

Similar to humans, fish make use of the otolith organs of the inner ear to transduce whole body accelerations with respect to gravity. The visual and tactile senses are mainly used to signal translational movement with respect to an external visual landmark or to sense contact slippage with a substrate. However, fishes have an additional network of mechanosensors distributed along the length of the body called the “lateral line”. The lateral line system contains between 100 to over 1000 sense organs, so-called neuromasts that are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail. Superficial neuromasts are located on the skin in direct contact with the flow while canal neuromasts exist in sub-epidermal canals connecting pore openings on the skin surface [12].

Fig. 1. Superficial and Canal Neuromasts of the Lateral Line (9).

Fig. 2. Superficial neuromast with 4 hair cells. Each hair cell has a staircase arrangement of cilia (9).

Fig. 3. Stair-case arrangement of stereocilia (10).

Each of these neuromasts contains multiple columnar hair cells embedded in nervous tissue and capped by a gelatinous cupula. Each of these hair cells has a protruding bundle of short stereocilia arranged in a stair-case step fashion and a single kinocilium adjacent to the talleststereocilium. The stereocilia and kinocilium form synaptic connections with the nervous tissue at the basal end and project into a potassium ion rich endolymph. The stereocilia are connected together by filamentous tip links joining the tip of a lower stereocilia to the side of the adjacent higher one. These connection points are locations of ion channels that preferentially admit potassium ions of the endolymph. When a fish encounters flow relative to its surface the surrounding cupula encounters a drag force proportional to the flow velocity and thus deflects in one direction. This stimulus displaces the stereocilia towards the kinocilium and elongates the tip link, thus opening the gate of the ion channel and admitting potassium ions that depolarises the cell. When stereocilia are displaced in the opposite sense the tension in the tip link is reduced and mechanical ion gate is shut, thus repolarising the cell [12]. The firing response of the nerve cells has been shown to vary with the cosine of the flow direction angle relative to the staircase axis. Consequently, based on the relative signals from different hair cells orientated in different directions the fish can get an exact image of the flow direction and velocity.

Fig. 4. Depolarization of hair cell due to flow-coupled deflection of stereocilia (11).

Now, superficial neuromasts occur individually or in rows on the fish skin and possesses between 4 and 15 hair cells each with their own stair case arrangement of cilia. The cupula extends directly into fluid flow around the fish and since the drag exerted on the cupula is a function of velocity, superficial neuromasts act as flow velocity detectors. Superficial neuromasts respond best to same direction flows up to 20 Hz alternating flows and thus serve behaviours depending on large-scale stimuli such as upstream orientation to bulk water flow and overall flow rate measuring.

Canal neuromasts are located between pores inside fluid filled canals under the skin and are sensitive to pressure gradients over broad range of stimulus frequencies. The simplest form is a straight-sided tube with water movements within the canal driven by the pressure difference between adjacent pore openings. By Bernouilli’s principle, faster flow will have lower pressure such that flow velocity in the canal is proportional to net acceleration between fish and surrounding water. The flow inside the canals is impeded by frictional forces of the boundary layer such that high inertial forces are required before any fluid motion occurs. Consequently, the canal neuromasts function as high-pass filters to attenuate the sensitivity to low frequency noise and respond best to rapid AC flows between 30-100 Hz.

Fig. 5. Canal Neuromast and Surface Pore (12).

Thus, the lateral line appears to consist of two subsystems that divide the frequency spectrum into low frequencies and higher frequency stimuli:

● A system of velocity-sensitive superficial neuromasts that responds to slow, uniform motions and that integrates large scale stimuli at the periphery such as constant currents and

●    A system of acceleration- or pressure-gradient-sensitive canal neuromasts that responds to rapidly changing motions and gives the fish the opportunity to orient towards sources such as prey or optimize swimming speed or tail-flapping frequency.

The next post will discuss the methods in which fishes take advantage of the the flow information provided by the neuromasts in order to “morph” their skins and locomotive behaviour for drag reduction.