Primary Instability Research Articles

Toward Cost-Effective Boundary Layer Transition Computations With Large-Eddy Simulation

An efficient large-eddy simulation (LES) approach is investigated for laminar-to-turbulent transition in boundary layers. This approach incorporates the boundary-layer stability theory. Primary instability and subharmonic perturbations determined by the boundary-layer stability theory are assigned as forcing at the inlet of the LES computational domain. This LES approach reproduces the spatial development of instabilities in the boundary layer, as observed in wind tunnel experiments. Detailed linear growth and nonlinear interactions that lead to the H-type breakdown are well captured and compared well to previous direct numerical simulation (DNS). Requirements in the spatial resolution in the transition region are investigated with connections to the resolution in turbulent boundary layers. It is shown that the subgrid model used in this study is apparently dormant in the overall transitional region, allowing the right level of the growth of small-amplitude instabilities and their nonlinear interactions. The subgrid model becomes active near the end of the transition where the length scales of high-order instabilities become smaller in size compared to the given grid resolution. Current results demonstrate the benefit of the boundary-layer forcing method for the computational cost reduction.

Secondary crossflow instability through global analysis of measured base flows

A combined experimental and numerical approach to the analysis of the secondary stability of realistic swept-wing boundary layers is presented. Global linear stability theory is applied to experimentally measured base flows. These base flows are three-dimensional laminar boundary layers subject to spanwise distortion due to the presence of primary stationary crossflow vortices. A full three-dimensional description of these flows is accessed through the use of tomographic particle image velocimetry (PIV). The stability analysis solves for the secondary high-frequency modes of type I and type II, ultimately responsible for turbulent breakdown. Several pertinent parameters arising from the application of the proposed methodology are investigated, including the mean flow ensemble size and the measurement domain extent. Extensive use is made of the decomposition of the eigensolutions into the terms of the Reynolds–Orr equation, allowing insight into the production and/or destruction of perturbations from various base flow features. Stability results demonstrate satisfactory convergence with respect to the mean flow ensemble size and are independent of the handling of the exterior of the measurement domain. The Reynolds–Orr analysis reveals a close relationship between the type I and type II instability modes with spanwise and wall-normal gradients of the base flow, respectively. The structural role of the in-plane velocity components in the perturbation growth, topology and sensitivity is identified. Using the developed framework, further insight is gained into the linear growth mechanisms and later stages of transition via the primary and secondary crossflow instabilities. Furthermore, the proposed methodology enables the extension and enhancement of the experimental measurement data to the pertinent instability eigenmodes. The present work is the first demonstration of the use of a measured base flow for stability analysis applied to the swept-wing boundary layer, directly avoiding the modelling of the primary vortices receptivity processes.

Patellar Instability in the Skeletally Immature.

This review will focus on the evaluation and management of patellar instability in the developing patient. A large number of surgical techniques have been described to prevent recurrent patellofemoral instability in the pediatric population, including both proximal and distal realignment procedures. The wide variety of treatment options highlights the lack of agreement as to the best surgical approach. However, when a comprehensive exam and workup are paired with a surgical plan to address each of the identified abnormalities, outcomes are predictably good. Patellar instability is a common knee disorder in the skeletally immature patient that presents a unique set of challenges. Rates of re-dislocation in pediatric and adolescent patients are higher than in their adult counterparts. Careful consideration of the physeal and apophyseal anatomy is essential in these patients. While the majority of primary patellar instability events can be treated conservatively, multiple events often require surgical intervention.

Effects of planar shear on the three-dimensional instability in flow past a circular cylinder

A Floquet stability analysis has been carried out in order to investigate how a planar shear in wake flow affects the three-dimensional (3D) instability in the near-wake region. We consider a circular cylinder immersed in a freestream with planar shear. The cylinder was implemented in a Cartesian grid system by means of an immersed boundary method. Planar shear tends to promote the primary instability, known as Hopf bifurcation where steady flow bifurcates into time-periodic flow, in the sense that its critical Reynolds number decreases with increasing planar shear. The effects of planar shear on the 3D instability are different depending on the type of 3D instability. The flow asymmetry caused by the planar shear suppresses a QP-type mode but generates a C-type mode. The conventional A and B modes are stabilized by the planar shear, whereas mode C is intensified with increasing shear. The criticality of each 3D mode is discussed, and the neutral stability curves for each 3D mode are presented. The curren...

Three-dimensional doubly diffusive convectons: instability and transition to complex dynamics

Three-dimensional doubly diffusive convection in a closed vertically extended container driven by competing horizontal temperature and concentration gradients is studied by a combination of direct numerical simulation and linear stability analysis. No-slip boundary conditions are imposed on all six container walls. The buoyancy number $N$ is taken to be $-1$ to ensure the presence of a conduction state. The primary instability is subcritical and generates two families of spatially localized steady states known as convectons. The convectons bifurcate directly from the conduction state and are organized in a pair of primary branches that snake within a well-defined range of Rayleigh numbers as the convectons grow in length. Secondary instabilities generating twist result in secondary snaking branches of twisted convectons. These destabilize the primary convectons and are responsible for the absence of stable steady states, localized or otherwise, in the subcritical regime. Thus all initial conditions in this regime collapse to the conduction state. As a result, once the Rayleigh number for the primary instability of the conduction state is exceeded, the system exhibits an abrupt transition to large-amplitude relaxation oscillations resembling bursts with no hysteresis. These numerical results are confirmed here by determining the stability properties of both convecton types as well as the domain-filling states. The number of unstable modes of both primary and secondary convectons of different lengths follows a pattern that allows the prediction of their stability properties based on their length alone. The instability of the convectons also results in a dramatic change in the dynamics of the system outside the snaking region that arises when the twist instability operates on a time scale faster than the time scale on which new rolls are nucleated. The results obtained are expected to be applicable in various pattern-forming systems exhibiting localized structures, including convection and shear flows.

What's New in Pediatric Upper Extremity Sports Injuries?

Sports injuries are common in pediatric and adolescent patients and the evaluation and treatment of these injuries continues to evolve. The purpose of this review is to provide a comprehensive appraisal of the most recent literature, highlighting updates on sports-related upper extremity injuries in pediatric patients. An electronic search of the PubMed, EMBASE, and Google Scholar databases was performed for keywords related to pediatric overhead athletes, osteochondritis dissecans (OCD) of the capitellum, medial epicondyle fractures, shoulder instability, and clavicle fractures. Search results were filtered by publication date to yield articles published electronically or in print on or after January 1, 2013 to May 30, 2017. Papers were selected based on expert opinion and consensus by the authors and included if deemed to have contributed important findings to the above topics. A total of 51 articles were deemed to have contributed significant findings to the literature: 11 overhead athlete, 9 OCD of the capitellum, 6 medial epicondyle fractures, 17 shoulder instability, and 8 clavicle fractures. The level of evidence for most studies was either Level III or IV. Overuse and traumatic conditions of the pediatric elbow including UCL tear, capitellar OCD, and medial epicondyle fractures represent a significant portion of injuries in the overhead athlete. Research in the prevention and treatment of primary and recurrent shoulder instability in young athletes continues to evolve. The operative treatment of clavicle fractures in adolescents has been increasing without a commensurate increase in the level of evidence supporting such treatment. Advances have been made in the treatment of sports-related upper extremity injuries in pediatric patients, however, high-level, comparative outcomes research in many areas is lacking and this review may help inform topics for future study. Level IV-Literature review.

Glenoid Bone Loss in Posterior Shoulder Instability: Prevalence and Outcomes in Arthroscopic Treatment

Background: Glenoid bone loss is a well-accepted risk factor for failure after arthroscopic stabilization of anterior glenohumeral instability. Glenoid bone loss in posterior instability has been noted relative to its existence in posterior instability surgery. Its effect on outcomes after arthroscopic stabilization has not been specifically evaluated and reported. Purpose: The purpose was to evaluate the presence of posterior glenoid bone loss in a series of patients who had undergone arthroscopic isolated stabilization of the posterior labrum. Bone loss was then correlated to return-to-duty rates, complications, and validated patient-reported outcomes. Study Design: Case-control study; Level of evidence, 3. Methods: A retrospective review was conducted at a single military treatment facility over a 4-year period (2010-2013). Patients with primary posterior instability who underwent arthroscopic isolated posterior labral repair were included. Preoperative magnetic resonance imaging was used to calculate posterior glenoid bone loss using a standardized “perfect circle” technique. Demographics, return to duty, complications, and reoperations, as well as outcomes scores including the Single Assessment Numeric Evaluation and the Western Ontario Shoulder Instability Index (WOSI) scores, were obtained. Outcomes were analyzed across all patients based on percentage of posterior glenoid bone loss. Bone loss was then categorized as below or above the subcritical threshold of 13.5% to determine if bone loss effected outcomes similar to what has been shown in anterior instability. Results: There were 43 consecutive patients with primary, isolated posterior instability, and 32 (74.4%) completed WOSI scoring. Mean follow-up was 53.7 months (range, 25-82 months) The mean posterior glenoid bone loss was 7.3% (0%-21.5%). Ten of 32 patients (31%) had no appreciable bone loss. Bone loss exceeded 13.5% in 7 of 32 patients (22%), and 2 patients (6%) exceeded 20% bone loss. Return to full duty or activity was nearly 90% overall. However, those with >13.5%, subcritical glenoid bone loss, were statistically less likely to return to full duty (relative risk = 1.8), but outcomes scores, complications, and revision rates were otherwise not different in those with no or minimal bone loss versus those with more significant amounts. Conclusion: Posterior glenoid bone loss has not previously been evaluated independently relative to patients with shoulder instability repairs. Sixty-nine percent of our patients had measurable bone loss, and 22% had greater than 13.5%, or above subcritical bone loss. While these patients were statistically less likely to return to full duty, the reoperation rate, complications, and patient-reported outcomes between groups were not different.

Short Stems Versus Conventional Stems in Cementless Total Hip Arthroplasty: A Long-Term Registry Study

BackgroundShort stems in total hip arthroplasty (THA) have recently gained increasing popularity, allowing mini-invasive exposures and bone-sparing approaches. However, long-term studies and recommendations for the routine use are not available. The aim of this report was to compare the survival rates and the reasons for revision of short stems versus conventional stems in cementless THAs, in a registry-based population. MethodsThe Registry of Prosthetic Orthopedic Implants (RIPO) was inquired about cementless THAs performed since 2000 to 2016. The stems were divided into short (<12 cm) and conventional ones, and then, classified according to the classification by Feyen and Shimmin: short stems with neck-retaining osteotomy (group A: 1684 hips), short stems with standard osteotomy (group B: 2727 THAs), and conventional stems (group C: 57,359 cases). Demographics, survivorships, and reasons for revision were investigated and compared. ResultsShort stems were preferentially implanted in younger patients and normal morphologies. Short and conventional stems showed comparable survival rates at long-term follow-up (>90% at 15 years). The rates of stem aseptic loosening, intraoperative fractures, and periprosthetic fractures were similar in the 3 groups. Group B had higher rates of revisions due to primary instability (early dislocations and impingement-related events; P < .05). Revisions due to pain were nonsignificantly higher in group B. ConclusionShort stems are reliable implants at long-term follow-up. The comparison with conventional stems showed no additional risk of premature aseptic loosening and intraoperative and periprosthetic fractures. However, the high rate of revisions due to pain and, mostly, primary instability should be investigated in clinical trials.

The cost-effectiveness of the arthroscopic Bankart versus open Latarjet in the treatment of primary shoulder instability

The purpose of this study was to conduct a cost-effectiveness analysis of the arthroscopic Bankart and the open Latarjet in the treatment of primary shoulder instability. This cost-effectiveness study used a Markov decision chain and Monte-Carlo simulation. Existing literature was reviewed to determine the survivorship and complication rates of these procedures. Health utility states (EQ-5D and quality-adjusted life-years) of the Bankart and Latarjet were prospectively collected. Using these variables, the Monte-Carlo simulation was modeled 100,000 times. In reviewing the literature, the overall recurrence rate is 14% after the arthroscopic Bankart and 8% after the open Latarjet. Postoperative health utility states were equal between the 2 procedures (mean EQ-5D, 0.930; P = .775). The Monte-Carlo simulation showed that the Bankart had an incremental cost-effectiveness ratio of $4214 and the Latarjet had an incremental cost-effectiveness ratio of $4681 (P < .001). Both the arthroscopic Bankart and open Latarjet are highly cost-effective; however, the Bankart is more cost-effective than the Latarjet, primarily because of a lower health utility state after a failed Latarjet. Ultimately, the clinical scenario may favor Latarjet (ie, critical glenoid bone loss) in certain circumstances, and decisions should be made on a case by case basis.

Onset of oscillatory Rayleigh-Bénard magnetoconvection with rigid horizontal boundaries

We present the results of linear stability analysis of oscillatory Rayleigh-Bénard magnetoconvection with rigid and thermally conducting boundaries. We have investigated two types of horizontal surfaces: (i) electrically conducting and (ii) boundaries which do not allow any outward current normal to the surface (magnetic vacuum conditions). For the case of electrically conducting boundaries, the critical Rayleigh number Rao(Q,Pr,Pm), the critical wave number ko(Q,Pr,Pm), and the frequency at the instability onset ω(Q,Pr,Pm) increase as the Chandrasekhar number Q is raised for fixed non-zero values of thermal Prandtl Pr and magnetic Prandtl number Pm. For small values of Pr, the frequency of oscillation ω at the primary instability shows a rapid increase with Pm for very small values of Pm followed by a decrease at relatively larger values of Pm. In the limit of Pr→0, Rao and ko are found to be independent of Q. However, the frequency ω increases with Q, but decreases with Pm in this limit. The oscillatory instability is possible at the onset of magnetoconvection if and only if Chandrasekhar's criterion is valid (i.e., Pm &amp;gt; Pr) and Q is raised above a critical value Qc(Pr,Pm) such that the product Pm*Qc≈91 for large Pm. For the stellar interior of an astrophysical body (Pm≈10−4 and Pr≈10−8), the value of this product Pm*Qc≈230. The boundary conditions for magnetic vacuum change the critical values of Rayleigh number, wave number, and frequency of oscillation at the onset. The oscillatory magnetoconvection occurs in this case, if Q&amp;gt;Qc, where Pm*Qc≈42 for large Pm. For steller interior, this value is approximately 64. A low-dimensional model is also constructed to study various patterns near the onset of oscillatory convection for rigid, thermally and electrically conducting boundaries. The model shows standing and drifting fluid patterns in addition to flow reversal close to the onset of magnetoconvection.

Internal flow effect on the parametric instability of deepwater drilling risers

This study attempts to explore and distinguish the effects of internal flow induced centrifugal force and Coriolis force on the parametric instability of deepwater drilling risers. The governing equation of a fluid-conveying flexible riser subjected to parametric excitations is established at first, which is then reduced into a system of first-order linear equations by use of the Galerkin method with attention to the Coriolis-related skew-symmetric matrix. The stability charts of a deepwater drilling riser at different internal flow velocities are plotted by applying the Floquet theory, and the effects of the centrifugal force and Coriolis force on the primary instability have been identified, distinguished and exemplified. Owing to the centrifugal force effect which can reduce the effective tension and decrease the natural frequencies of the riser, the simple instability zones are enlarged and shifted in the stability chart, and more modes can be triggered in the chosen excitation domain. On the other hand, the simple resonance zones become narrower ascribed to the Coriolis force's damping effect. The most notable finding is that the Coriolis force creates and/or strengthens the coupling effect of excited modes, which can make the combination resonance much more profound.

Transition to turbulence in the rotating-disk boundary-layer flow with stationary vortices

This paper proposes a resolution to the conundrum of the roles of convective and absolute instability in transition of the rotating-disk boundary layer. It also draws some comparison with swept-wing flows. Direct numerical simulations based on the incompressible Navier–Stokes equations of the flow over the surface of a rotating disk with modelled roughness elements are presented. The rotating-disk flow has been of particular interest for stability and transition research since the work by Lingwood (J. Fluid Mech., vol. 299, 1995, pp. 17–33) where an absolute instability was found. Here stationary disturbances develop from roughness elements on the disk and are followed from the linear stage, growing to saturation and finally transitioning to turbulence. Several simulations are presented with varying disturbance amplitudes. The lowest amplitude corresponds approximately to the experiment by Imayama et al. (J. Fluid Mech., vol. 745, 2014a, pp. 132–163). For all cases, the primary instability was found to be convectively unstable, and secondary modes were found to be triggered spontaneously while the flow was developing. The secondary modes further stayed within the domain, and an explanation for this is a proposed globally unstable secondary instability. For the low-amplitude roughness cases, the disturbances propagate beyond the threshold for secondary global instability before becoming turbulent, and for the high-amplitude roughness cases the transition scenario gives a turbulent flow directly at the critical Reynolds number for the secondary global instability. These results correspond to the theory of Pier (J. Engng Maths, vol. 57, 2007, pp. 237–251) predicting a secondary absolute instability. In our simulations, high temporal frequencies were found to grow with a large amplification rate where the secondary global instability occurred. For smaller radial positions, low-frequency secondary instabilities were observed, tripped by the global instability.

Structural Transformations in Nematic Liquid Crystals with a Hybrid Orientation

The structural transformations in a nematic liquid crystal (NLC) layer with a hybrid orientation (planar director orientation is created on one substrate and homeotropic director orientation is created on the other) are studied. In the case of a dc voltage applied to the NLC layer, the primary instability is flexoelectric. It causes the appearance of flexoelectric domains oriented along the director on the substrate with a planar orientation. When the voltage increases further, an electroconvective instability in the form of rolls moving almost normal to flexoelectric domains develops along with these domains. Thus, the following spatially periodic structures of different natures coexist in one system: equilibrium static flexoelectric deformation of a director and dissipative moving oblique electroconvection rolls. The primary instability in the case of an ac voltage is represented by electroconvection, which leads to moving oblique or normal rolls depending on the electric field frequency. Above the electroconvection threshold, a transition to moving “abnormal” rolls is detected. The wavevector of the rolls coincides with the initial director orientation on the substrate with a planar orientation, and the projection of the director at the midplane of the NLC layer on the layer plane makes a certain angle with the wavevector. The results of numerical calculations of the threshold characteristics of the primary instabilities agree well with the obtained experimental data.

Operative shoulder instability injury management in Australian Football League players: A case series

ObjectivesThe purpose of this study was to review the surgical management procedures of shoulder instability injuries in Australian Football League (AFL) players, and determine outcomes regarding return to sport and injury recurrence. DesignRetrospective cohort study. MethodsElite AFL players with shoulder instability resulting in surgery were assessed in a retrospective cohort design (72 players/77 shoulders). Type of initial injury, surgical management, return to sport and injury recurrence were obtained. The mean follow-up period post-surgery was 2.9 years. Return to sport outcomes were compared between arthroscopic and open surgery using a Kaplan–Meier survival analysis. Logistic regression modelling was used to determine associations between injury recurrence, type of injury, participant age and method of surgery. ResultsShoulder instability injuries occurred most frequently during tackling (40%). Arthroscopic surgery was preferred for primary shoulder instability. Nine (16%) recurrences occurred in those who underwent arthroscopic surgery compared to two (9%) following open surgery. Return to the elite level was slightly but significantly (2 weeks, p=0.049) longer for open compared to arthroscopic surgery. Recurrence was 5 times more likely if the primary injury was a dislocation and more likely in players who were younger at the time of surgery. ConclusionTackling was the predominant mechanism for shoulder instability injuries in AFL players and arthroscopic surgery was more commonly performed for primary injuries. Sustaining a dislocation as the primary injury and younger age increased the likelihood of recurrent instability. Careful consideration should be given to the operative management of these individuals.

Parametric instability of a functionally graded cylindrical thin shell subjected to both axial disturbance and thermal environment

This paper focuses on the parametric instability of a functionally graded (FG) cylindrical thin shell under both axial disturbance and thermal environment. Based on Love's thin shell theory, and considering the temperature-dependent properties of FG cylindrical shell, the dynamic equations of the FG cylindrical shell are derived by Hamilton's principle. The multiple scales method is performed to obtain the instability boundaries of the shell with axial disturbance. The primary and combination instabilities of the shell are studied systematically. Moreover, numerical simulations are utilized to discuss the influences of axial disturbed amplitude, material heterogeneity and thermal effects on instability regions, frequency characteristics of the shell. Specially, some numerical results are given to illustrate the combined influence of axial disturbed amplitude and temperature variation on instability regions.

Ground effects on the stability of separated flow around a NACA 4415 airfoil at low Reynolds numbers

We perform a linear BiGlobal modal stability analysis on the separated flow around a NACA 4415 airfoil at low Reynolds numbers (Re=300–1000) and a high angle of attack (α=20°), with a focus on the effect of the airfoil's proximity to two different types of ground: a stationary ground and a moving ground. The results show that the most dominant perturbation is a Kelvin–Helmholtz mode, which gives rise to a supercritical Hopf bifurcation to a global mode, leading to large-scale vortex shedding at a periodic limit cycle. As the airfoil approaches the ground, this mode can become more unstable or less unstable, depending on the specific type of ground: introducing a stationary ground to an otherwise groundless system is destabilizing but introducing a moving ground is stabilizing, although both effects weaken with increasing Re. By performing a Floquet analysis, we find that short-wavelength secondary instabilities are damped by a moving ground but are amplified by a stationary ground. By contrast, long-wavelength secondary instabilities are relatively insensitive to ground type. This numerical–theoretical study shows that the ground can have an elaborate influence on the primary and secondary instabilities of the separated flow around an airfoil at low Re. These findings could be useful for the design of micro aerial vehicles and for improving our understanding of natural flyers such as insects and birds.

Layer formation in horizontally forced stratified turbulence: connecting exact coherent structures to linear instabilities

We consider turbulence in a stratified ‘Kolmogorov’ flow, driven by horizontal shear in the form of sinusoidal body forcing in the presence of an imposed background linear stable stratification in the third direction. This flow configuration allows the controlled investigation of the formation of coherent structures, which here organise the flow into horizontal layers by inclining the background shear as the strength of the stratification is increased. By numerically converging exact steady states from direct numerical simulations of chaotic flow, we show, for the first time, a robust connection between linear theory predicting instabilities from infinitesimal perturbations to the robust finite-amplitude nonlinear layered state observed in the turbulence. We investigate how the observed vertical length scales are related to the primary linear instabilities and compare to previously considered examples of shear instability leading to layer formation in other horizontally sheared flows.

Effects of second diffusing component and cross diffusion on primary and secondary thermoconvective instabilities in couple stress liquids

Linear and weakly nonlinear analyses are made for the Rayleigh-Benard convection in two-component couple-stress liquids with the Soret effect. Conditions for pitchfork, Hopf, Takens-Bogdanov, and codimension-two bifurcations are presented. The Lorenz model is used to study the inverted bifurcation. Positive values of the Soret coefficient favor a pitchfork bifurcation, whereas negative values favor a Hopf bifurcation. Takens-Bogdanov and codimension-two bifurcations are not as much influenced by the Soret coefficient as pitchfork and Hopf bifurcations. The influence of the Soret coefficient on the inverted bifurcation is similar to the influence on the pitchfork bifurcation. The influence of other parameters on the aforementioned bifurcations is also similar as reported earlier in the literature. Using the Newell-Whitehead-Segel equation, the condition for occurrence of Eckhaus and zigzag secondary instabilities is obtained. The domain of appearance of Eckhaus and zigzag instabilities expands due to the presence of the Soret coefficient for positive values. The Soret coefficient with negative values enhances heat transport, while positive values diminish it in comparison with heat transport for the case without the Soret effect. The dual nature of other parameters in influencing heat and mass transport is shown by considering positive and negative values of the Soret coefficient.

The effect of adsorption modeling on the stability of surfactant-laden liquid film flow

The primary instability of gravity-driven, liquid film flow is examined when the liquid contains a soluble surfactant. It is shown that the model adopted for the description of adsorption equilibrium affects significantly the prediction of the critical Reynolds number, \(Re_\mathrm{c} \). Weak attractive/repulsive van der Waals forces between adsorbed molecules stabilize/destabilize the film, whereas strong, attractive van der Waals forces result in non-monotonic variation of \(Re_\mathrm{c} \) with surface coverage. Ionic surfactants form an electric double layer at the interface, whose repulsive potential destabilizes the film. The intensity of electrostatic effects varies inversely with the solution ionic strength, and two asymptotic limits—zero and high concentration of an indifferent salt—are examined. The competition between electrostatic and van der Waals forces results in a very rich behavior, with the critical Reynolds number increasing or decreasing with surface coverage. The parametric variation of \(Re_\mathrm{c} \) is physically understood in terms of the effectiveness of surfactant mass exchange (between the interface and the bulk) in short-circuiting the basic stabilization mechanism, i.e., Marangoni stresses.

Mechanochemical pattern formation in simple models of active viscoelastic fluids and solids

The cytoskeleton of the organism Physarum polycephalum is a prominent example of a complex active viscoelastic material wherein stresses induce flows along the organism as a result of the action of molecular motors and their regulation by calcium ions. Experiments in Physarum polycephalum have revealed a rich variety of mechanochemical patterns including standing, traveling and rotating waves that arise from instabilities of spatially homogeneous states without gradients in stresses and resulting flows. Herein, we investigate simple models where an active stress induced by molecular motors is coupled to a model describing the passive viscoelastic properties of the cellular material. Specifically, two models for viscoelastic fluids (Maxwell and Jeffrey model) and two models for viscoelastic solids (Kelvin–Voigt and Standard model) are investigated. Our focus is on the analysis of the conditions that cause destabilization of spatially homogeneous states and the related onset of mechano-chemical waves and patterns. We carry out linear stability analyses and numerical simulations in one spatial dimension for different models. In general, sufficiently strong activity leads to waves and patterns. The primary instability is stationary for all active fluids considered, whereas all active solids have an oscillatory primary instability. All instabilities found are of long-wavelength nature reflecting the conservation of the total calcium concentration in the models studied.